INSIVUMEH reported a gradual increase of tremor amplitude at Pacaya during 17-18 June. Observers noted that small ash ejections from Mackenney cone were dispersed around the crater. Tremor continued to be detected during 20-22 June. Ash emissions continued to be confined to the crater area during 21-22 June, and incandescence from the crater was visible at night.

Introduction. Pacaya, which in recent years has consistently erupted olivine-bearing high alumina basaltic lavas, erupted with remarkable violence on both 27 and 28 May 2010 with an explosion on the 27th lasting ~45 minutes. This was followed by a smaller explosion the next day that generated a plume assessed from satellite and meteorological data as reaching 13 km altitude. In this report we describe those events as explosions in order to distinguish them from the ongoing, decades-long, and often effusive eruption generally seen at Pacaya. The terms ‘explosion’ and ‘explosive’ appear warranted given such factors as the suddenness of escalation, the ~13 km plume altitude (~10 km over the summit when measured during the weaker explosion on the 28th, the density of projectiles, and the scale of the tephra fall. The term explosion seems consistent with common practice (Sparks, 1986; Fiske and others, 2009).

The following report emphasizes Pacaya’s behavior in 2010, including the 27 and 28 May explosions and impacts continuing into early June 2010. Our last report (BGVN 34:12) discussed behavior into mid-January 2010. Some of the reporting came from reports of Guatemalan agencies (eg. INSIVUMEH and CONRED, acronyms spelled out in the Information contacts section at bottom), newspapers (eg. Prense Libra, 2010a, b), videos and photos, and cited manuscripts and papers. It especially benefited from a draft manuscript prepared by Rüdiger Escobar Wolf (REW, 2014) and graciously provided to Bulletin editors. REW also provided reviews, insights, and numerous tailored graphics but bears no responsibility for possible errors induced by Bulletin editors.

The explosions were preceded months to weeks earlier by extra-crater venting of lava flows on the E and SE flanks. The lava flows covered substantial areas after emerging effusively at two widely spaced vents in atypical extra-crater or crater-margin locations.

Following the Introduction, this report’s subsections address the following topics: (1) the Guatemalan hazard agency CONRED’s reports, (2) a sample of available video and photo documentation of Pacaya’s behavior, (3) events prior to the 27 May explosion, (4) the explosions and some of the impacts, (5) the seismic record showing the pattern of escalation around the time of the explosions, (6) a brief summary of the critical initial aviation reports, and (7) a geotechnical slope stability study that suggests gravitational instability at Pacaya, particularly owing to the cone’s magma pressure and seismic loading.

Pacaya , which has a record of eruptions dating back over 1,600 years, has been erupting the majority of the time since 1961, often emitting rough-surfaced lavas but also occasionally discharging explosions. The centerpiece of the National Park of the same name, it is the most often climbed volcano in Guatemala. There have been 69 prior Smithsonian-published reports describing behavior from 1969 to early January 2010 (CSLP 03-70 to BGVN 34:12). REW (2013) ranked the 27 May explosions as sub-plinean and the associated lava emissions as the largest since similar events in 1961.

Figure 42. (Top) A map showing Pacaya’s location in Central America. (Bottom) A map emphasizing Pacaya’s location with respect to the central portion of Guatemala City (red square labeled ‘Guatemala’). The larger combined urban area associated with that Capital city stretches well beyond the square symbol and contains ~3.5 million residents. AmatitlÁn was heavily damaged by Pacaya’s May 2010 ashfall and the knock-on effects of Tropical Storm Agnes that arrived two days later. Top map taken from Morgan and others (2012); bottom map revised from a base map found online at Ezilon Maps.

The larger tephra blanket spread N, covering an area of more than 1,000 km2 including the bulk of the Guatemala City metropolitan area, the largest city in Central America, population ~3.5 million. The City’s center lies ~25 km NNE of Pacaya’s summit but a 5-km-wide strip of urban and suburban development now stretches from its older core (red square, figure 42)to ~9 km N of the summit. The tephra shut down La Aurora, the county’s primary international airport and among the region’s busiest, for 5 consecutive days.

The 27 May 2010 explosion destroyed or damaged nearly 800 houses in nearby communities, forcing ~2,000 residents to evacuate and injuring 59 people. A high density of ballistics fell on nearby hamlets and villages, particularly those 2.5-3.5 km N of the MacKenny cone (El Cedro, San Francisco de Sales, and Calderas). The ballistics had sufficient mass and velocity to puncture roofs with a density on the order of one puncture per square meter in some places. Many more smaller ballistics bent but did not penetrate the corrugated sheet metal roofs common in many of the region's dwellings. Some of the ballistics were sufficiently hot to start fires.

Ash caused widespread damage locally, and up to ~8 cm of ash fell on parts of metropolitan Guatemala City, the nation’s capital, centered ~35 km NNW of Pacaya. Up to 20 cm of tephra accumulated at and near Pacaya. According to available census data, the population within 10 km of Pacaya was 57,000 (John Ewert, USGS-CVO, personal communication).

Accounts from Guatemalan meteorological stations reported that detectable ash from the 2010 explosions fell as far away as the Caribbean coast. Brianna Hetland was both a graduate student in volcanology and a US Peace Corps Volunteer in Guatemala during 2010-2012. Hetland noted in a message that she had spoken with another Volunteer who said ash had blanketed his neighborhood near Coban (in Samac, Alta Verapaz) ~180 m N of Pacaya (figure 42, bottom). Hetland documented post-eruptive conditions at Pacaya, composed a blog on the impact and clean up, and gave a talk on those aspects as well as multifaceted monitoring conducted by fellow students and faculty at Michigan Technological University (Hetland, 2012a, b; Walikainen, 2010).

Some of the impacts of the freshly fallen ash were amplified and other impacts were diminished by heavy rains and flooding due to Tropical Storm Agatha that struck the region 2 days later, with some areas receiving 0.9 m of rain. The floodwater run carried ash that dislodged debris, clogged drainage systems, left thick deposits on valley floors, and damaged many bridges. The scale of the combined disasters led to more analysis of hardships, mitigation, and economic impact than usual at many eruptions, as exemplified by the detailed assessments by Wardman and others (2012). Those authors visited in the aftermath from New Zealand in order to study impacts that might be analogous to hazards elsewhere. They found that one moderating impact of the rain was to cee crops, which were washed clean of ash and residual acids. The authors also found that that a prompt and efficient cleanup was initiated by the Capital municipality to remove tephra from the 2,100 km of roads in the Capital. An estimated 11,350,000 m3 of tephra was removed from the city’s roads and rooftops.

Diminishing strombolian activity and lava flows in the crater area continued into at least late June 2010. By this time the emissions had become more like the generally effusive decades-long eruption, which was still ongoing when this was written in late 2014. In addition to the information here, Pacaya’s discharge rates have been summarized for the years 2004-2010 on the basis of infrared satellite images (Morgan and others, 2013). As would be expected, a strong peak in radiance developed in late May 2010.

REW (2013) noted one death attributed to the explosion and tephra fall and 179 deaths attributed to the Tropical Storm. Two people died at Pacaya days prior to the explosion of 27 May 2010. Wardman and others (2013) mentioned two further deaths due to people cleaning tephra from roofs.

Geochemical analysis of material erupted on 27 and 28 May is not yet reported. As background, Matías and others (2012) describe Pacaya’s recent lavas as all high-alumina basalts with SiO2 contents of 50-52.5 weight percent and MgO contents of 3-5 weight percent. Common phynocrysts (visible minerals) included plagioclase, olivine, and opaque minerals (Conway, 1995). There is a slight variation of CaO in this group of lavas, which suggests a phenocryst enrichment or depletion. The lava compositions have remained broadly similar since 1961, and for many previous lavas as well, although some more felsic compositions are represented at older flank eruptions (Eggers, 1971).

CONRED reports. Perspective on the disaster can be gained from the chronology and content of announcements issued by CONRED (the Guatemalan agency for disaster reduction; Coordinadora Nacional para la Reducción de Desastres, table 4). These will be referred to in text by “CONRED” followed by their bulletin number.

Table 4. A summary of key CONRED information bulletins issued relevant to Pacaya’s May 2010 eruption (http://conred.gob.gt/). After Escobar Wolf (2012) in addition to a similar table by Wardman and others (2012). Not all bulletins are included in this table.

Began to mobilize staff to villages near volcano around 1500 on the 27th, to discuss and implement pre-emptive evacuation. Seven shelters were prepared in San Vicente Pacaya to accommodate refugees.

When the paroxysmal phase of eruption started (after 1900), evacuation of villages to the W (El Rodeo and El Patrocinio) was already underway, however, tephra and ballistics were dispersed primarily to the N and the villages of El Cedro, San Francisco de Sales and Calderas were the most severely affected.

28 May 2010

731

Declared Red Alert. As of 1239 on the 28th over 1600 people had been evacuated from the villages of San Francisco de Sales, El Rodeo, El Patrocinio, El Cedro, Calderas, and Caracolito. They moved to San Vicente Pacaya.

Civil Aviation authorities closed La Aurora International Airport due to tephra fall. The Ministry of Education closed schools in Escuintla, Sacatepequez and Guatemala departments. Access to the National Park remained restricted.

The municipality-level response agency (with a similar name, COMRED, not CONRED) was activated in Villa Canales. It set up shelters in the municipal auditorium, a church, and the municipal hall.

In the afternoon at 1424 on the 28th, high eruptive vigor resumed and tephra again fell on Guatemala City, but in much smaller quantities than during the previous day.

29 May 2010

748

By this time, a total of 2635 people were in shelters due to the eruption; ~400 houses had been slightly damaged and 375, severely damaged.

27 May 2011

1673

One year later; a retrospective summary of civil defense responses to the eruption and the larger engulfing disaster, tropical storm Agatha.

Events prior to the energetic 27 May explosion. Figure 43 highlights Pacaya’s vent locations (1961 to 2009 vents as green dots), including the two new E and SE flank vents that emitted lava flows (red areas).

Figure 43. Simplified geological map of Pacaya, based on cited references, INSIVUMEH mapping, and GOES satellite data. The key at right calls attention to features such as the collapse scarp forming the N and E of margin of the main crater and the lava flows of prehistoric age (Eggers, 1969, 1972; Bonis, 1993) through about mid-2010. Migrating vents mapped during 1961-2012 (Matías, 2010; Rose and others, 2012) appear as dark-green dots (many clustered on or near the MacKenney cone’s summit). The red areas on the SE flank and E flank represent lava with the noted age constraints from REW’s analysis of satellite data. The SE flank vent had emitted by mid-2010 a field of lava approaching the size of the 1961 Cachiajinas lava flow (purple). The latter flow both vented and advanced within Pacaya’s collapse scarp. In contrast, the SE flank flow was the first in historical times to vent and flow outboard of the scarp. The cone residing on Pacaya’s NW rim, Cerro Chino, enters discussion frequently in this report. Note the depression (notch or trough) here labeled “New fissure like structure.” Map created and provided by REW.

Changes in eruption behavior preceded the 27-28 May explosions by several months.

From 2004 to around the end of 2009, Pacaya’s eruptive intensity was often low. A clear sign of changes took place in February 2010 when lava flows emerged at vents on the S and SE flanks (table 1). These vents sit well outboard of the usual points of lava emission, which have in recent decades been limited to spots within the central crater, an area bounded by a large engulfing collapse scarp (a Somma rim; Eggers, 1969; figure 43). The two previously mentioned deaths occurred on 18 April when, according to the news, they were hit by a rock avalanched caused by an explosion. By 17 May, SE flank lava flows had reached 1.5 km long and the Park began restricting access (table 1).

The scene on the SE flank appears in figure 44.

Figure 44. Pacaya’s SE flank eruption as seen during the day on 27 May 2010. The ultimate distribution of lavas appears on the preliminary map by REW (2014). Image courtesy of Gustavo Chigna (INSIVUMEH).

Earlier on the 27th (prior to the explosion), INSIVUMEH volcanologist Gustavo Chigna looked out over the crater area and counted at least 16 distinct vents emitting lava. Chigna was surprised, and his’s comment was something like, ‘It looked like water gushing out of a sieve.’ That scale of new extrusive sites helped alert authorities that the volcano’s behavior had escalated well beyond the norm and led to restricting public access to Pacaya.

During the 5 years prior to the 27 May 2010 explosion, sporadic vent openings limited to the MacKenney cone and adjacent areas (particularly the N crater) extruded lava flows (green dots, figure 43). Many of the resulting lava flows were each only active for periods of days to months. INSIVUMEH sometimes reported multiple simultaneous lava flows from distinct vents on the cone, which occurred, for example, during April 2009. Most of the lava was confined to the main crater or portions downslope and W of the E-bounding collapse scarp. The case in 2005 illustrated that the topographic boundary associated with the NE segment of the collapse scarp had diminished in places to the point where lava flows could cross the scarp (BGVN 33:08).

Around January 2010, Gustavo Chigna (INSIVUMEH) indicted the end of mainly lower effusive activity ongoing since 2004. The new upsurge fed several lava flows from vents on Pacaya’s main cone. In harmony with this comment, the video by Crossman (2009) indicates that on 24 December 2009 the volcano emitted considerable lava. Venting was effusive and at both the MacKenney cone’s summit and base. Visible plumes were nearly absent.

Table 5 lists a small sample of available videos taken at Pacaya that aid in documenting its behavior. The table includes videos taken before, during, or shortly after the 27 May explosion, with the two pre-explosion videos capturing behavior relevant to this subsection. The videos from other parts of the table are discussed in appropriate sections below.

Table 5. Some photos and videos that advance understanding of Pacaya behavior during December 2009 to about 2 June 2010 (a week after the explosion). The cases presented are a sample, not an exhaustive list. Compiled by Bulletin editors.

Video (V) or Photo (P) and source

Date acquired / Date posted if clearly stated)

Title; Content; URL

How cited in text of this report

V; Patrick R. Crossman

24 Dec 2009 / 24 Dec 2011

Title: “Hiking the Pacaya volcano in Guatemala”

This video chronicles a group visiting Pacaya amid ongoing effusive volcanism in comparatively calm conditions and with people in many scenes. Some parts of the video depict a narrow (1- to 2-m wide), channelized, slowly moving lava flow. That flow appears to vent near the base of the MacKenney cone, devoid of visible plume, and traverses a region of low incline. The path of the molten flow is sinuous rather than linear. The visitors roast marshmallows in radiant heat from the flows. The video also cuts to scenes at the MacKenney cone’s summit, where a larger flow several meters wide vents in a stable, effusive manner, also devoid of an associated plume.

[Date confirmed with Moon and by comparison to his dated still photos]

Title: “Pacaya Volcano, Guatemala [1080p HD]”

Close up views showing copious lava flowing down the E flank from the new vent there. Accompanies GPS record of hiking track and still photos. Music accompanies the video. Dovetails with a Landsat image from about a week earlier, which also documents the E flank lavas. See text for more discussion.

Title: “Raw video of damage caused by volcano eruptions in Guatemala and Ecuador”;

The video shows, for Pacaya, images of advancing lava flows and some distant views of the volcano in daylight with a moderate plume above it. There are many scenes of damage, evacuation, and human impact, including ash-loaded corrugated metal roofs that buckled; ash on airliners; brigades of people sweeping and carting off ash from city streets and an airport runway; and children sheltering in a relief center.

“In just the past seven days, residents of Guatemala and parts of neighboring Honduras and El Salvador have had to cope with a volcanic eruption and ash fall, a powerful tropical storm, the resulting floods and landslides, and a frightening sinkhole in Guatemala City that swallowed up a small building and an intersection. Pacaya volcano started erupting lava and rocks on May 27th, blanketing Guatemala City with ash, closing the airport, and killing one television reporter who was near the eruption. Two days later, as Guatemalans worked to clear the ash, Tropical Storm Agatha made landfall bringing heavy rains that washed away bridges, filled some villages with mud, and somehow triggered the giant sinkhole--the exact cause is still being studied. (34 photos total).”

Helicopter views of flight generally towards, and then at, Pacaya, which was still in eruption, with initial views showing Agua volcano and parts of Lake Amatitlán. Low weather clouds covered extensive areas. This video captured a decidedly non-vertical, denser black plume from Pacaya feeding a lighter, tan colored more massive plume that appears to drop ash as it is carried to ten’s of kilometers downwind (directed E-SE-S). Shots include those of Cerro Chino and antenna towers there, and widespread steaming on the MacKenney cone that coalesced into large steam clouds low over much of the central crater area.

(Narration by news reporter referring to explosion as 1 week ago, thus the ‘About 2 -3 June’ date in the previous column.) According to REW , this video shows lavas emitted at the new SE flank vent. Remarkable images, some seemingly shot from helicopter and others from the ground, showing copious channelized lava flows moving rapidly downslope to the SE. At the vent area there are three small vents discharging spatter from coalescing cones with very steep sides. Their glowing summit craters gave off occasional eruptions as well as occasional puffs of gases, glowing spatter, and possibly flames. Some shots show incandescent lava flows several kilometers long. Rising plumes sometimes display toroidal motion, rotational behavior reminiscent of dust devils.

Figure 45 shows one of several Landsat views of the E flank in an infrared image acquired on 23 March 2010. It showed high thermal radiance in a narrow linear thermal anomaly headed E outboard of the usual eruptions confined to the crater. The E-flank area is devoid of vegetation, which rules out a local fire there, meaning that the anomaly was due to a lava flow. The number of clear (cloud-free) views of Pacaya available during March through June was limited. REW plotted this anomaly in a KMZ file format (red line, figure 46).

Figure 45. A Landsat 7 thermal image of Pacaya on 23 March 2010 showing high heat flux as red. The small red area is on the MacKenney cone. The larger red area is a lava flow that had extended E. A site visit and video by Moon (2010) on 1 April (8 days later) confirmed lava flows on the order of 2-4 m wide. Black and marginal gray areas are older lava flows; green areas are vegetated with some cultivated or pasture land in shades of brown. This image contains artifacts in the form of gray diagonal stripes. The stripes are due to the failure of the Scan Line Corrector (SLC), which compensates for the satellite’s forward motion. Courtesy of REW.

Figure 46. A Google Earth view of the land surface looking radially outward (E) down Pacaya’s E flank (N is to the left). The red line indicates the location of the lava flow axis from heat flux in Landsat images. The flow’s source was at or very near the collapse scarp. The yellow line indicates the film crew’s 1 April 2010 excursion route recorded with GPS as they approached the lava flow, filmed it at close range, and then headed back towards the trailhead (Moon, 2010). For scale, the lava flow is ~0.3 km long. Graphic files, analysis, and compilation created and provided by REW.

The new E flank (extra-crater) lava flow documented by Landsat on 23 March was the subject of a video by Moon (2010) taken on 1 April (table 1; see their excursion route on figure 46). The footage was shot during daylight hours at high resolution [1080p HD] and later processed to obtain vibrant red, orange, and yellow colors.

The discharges were effusive and few visible emission clouds accompanied the lava flows seen in the video. A dark plume remained above the MacKenney cone’s summit.

As seen in fs 47-50, the lava documented by Moon (2010) in photo and video was several meters wide and passing over irregular terrain. As seen from a distance (eg. figure 47), some sectors of the flow’s channel stood well above the surrounding landscape. In the area visited, the lava remained confined behind jumbled but effective levees as it passed through and over the a’a (rough textured) flow field.

Figure 47. A 1 April 2010 photo of Pacaya’s E flank lava flow seen in the distance as it descends across an a’a flow field. Courtesy of H. Paul Moon (see table 5).

Figure 50. A still closer view of Pacaya’s E-flank lava (taken from just a few meters away), which was moving swiftly. In his YouTube notes on his teams 1 April 2010 visit Moon commented that “the heat was so intense that I could only hold out for brief shots, needing to turn away regularly to avoid getting scorched.” Courtesy of H. Paul Moon (see table 5).

Figure 51. An oblique Google Earth view of Pacaya looking roughly WNW. At left in orange appear the upslope areas of the fissures that fed the SE flank lava flows. Farther NE (to the right) appear another set of fresh black lavas that reside on the upper E flank. The green line traces high heat emissions REW found in Landsat imagery from 23 March 2010, the same lava flow that had been the subject of Moon’s video ~8 days later. Both sets of flows and vents were the first clearly documented to extend E of the collapse scarp in historical times. Analysis, compilation, and topographic files all provided by REW.

On 18 April 2010, according to a news report in the newspaper Prensa Libre, a Venezuelan tourist and her Guatemalan guide died on Pacaya. The news report stated the deceased were in the area of high risk when struck by material released from an explosion. Some of the other 14 people on the scene sustained injuries.

On 17 May 2010, observers saw abundant lava escaping from a new SE-flank vent (CONRED 708). A mound had formed at the vent area. The lava from this vent had by 17 May extended as far as 1.5 km. As seen on figure 43, the SE flank lava flows and their fissures ultimately fed lava flows trending roughly S for ~2.5 km then turning sharply (~90 degrees) to the W and extending in that direction another ~2.5 km.

CONRED 708 made a recommendation to the Pacaya National Park authority to restrict visitor access to the lava flows. The 17 May report noted that Pacaya’s activity was considered to be relatively high, but it left out language suggesting a crisis at this point. According to the press, access to the volcano was restricted following the recommendation.

On 17 May, the newspaper Prensa Libre featured an undated night photo of the MacKenney cone taken from the N, presumably of this stage of Pacaya’s eruption. It showed a dense spray of glowing material thrown from the MacKenney cone’s summit and rising hundreds of meters. The cone’s N rim contained a recently formed V-shaped notch (or trough). Out of that notch poured a broad lava flow. Several hundred meters down the MacKenney cone’s N face, the broad flow split into two flows descending the cone’s steep face on diverging paths. The notch in the cone stands out as a clear morphologic change associated with this time interval (~10 days prior to the 27 May explosion), and as will be seen below, it served as a conspicuous vent site for the fissure emissions documented during the explosions.

The day before the explosion, on 26 May, eruptive and seismic intensity both increased markedly. An eruptive plume reached 1 km above the vent and fine tephra fell on villages around the volcano (CONRED 726 on 26 May, table 1). CONRED recommended fully closing Pacaya National Park, and they warned aviation authorities of airborne ash near Pacaya. No call was yet made to evacuate residents living adjacent Pacaya.

Vigorous explosions starting 27 May 2010. Pacaya’s eruptive vigor increased to the point of strong strombolian eruption, with the initial increase noted on the 27th in a morning report in Prensa Libre. More intense explosions occurred at around 1500 when observers noted explosions discharging about once per second and saw glowing material thrown ~1.5 km above the crater, and taller rising dark clouds carrying finer tephra that dispersed over nearby villages.

The exact start time of the intense 27 May explosion is variously reported, but available visual observations suggested to REW (2014) that it was during the interval 1800-1900. CONRED 729 indicated the climax (the explosion)began at 1900. Seismic data, discussed in a subsection below underwent the highest (RSAM) amplitudes during 1730-1830 local time on the 27th. Aviation reporting of satellite data on eruptive plumes, discussed in a subsection below, was initially ineffectual for the 27th owing to above-lying weather clouds.

What is clear is that the explosion late in the day on the 27th drove forth intense fire fountaining and vigorous ejection of tephra and ballistics.

Figure 52 shows a broad fire fountain frame taken from a Youtube video posted on 28 May—but it lacked an acquisition date (RT news channel, 2010a). REW interprets this video as taken during the major climax (explosion) during the night on the 27th. The eruption was clearly of fissure style at this point but the upper extent of the glowing material was possibly masked by ash clouds. Some of the textures within the glowing region are explained in the f caption and in the text below.

Figure 52. A frame captured from a news video taken at night from Pacaya’s NNW side documenting powerful curtain-style emissions (fire fountains) from the main crater area (which includes the MacKenney cone). The foreground consists of the dark silhouette of Cerro Chino (indicated on figure 43). Some of the tall antenna towers there appear as narrow vertical dark streaks backlit by the brighter orange fire fountains. Many of the towers and radio shacks on the ground near their bases were destroyed. Taken from RT news (see table 5 (RT News, 2010 (a)).

REW described the video source for figure 52 as taken looking at Cerro Chino (indicated on figure 43) from at or near the town of El Cedro, ~3 km to the NNW of the vent. The diffuse zones of near darkness in the midst of the fountains are rising ash clouds locally diminishing the glow. Thus it is clear from the dynamics seen on the video, that the glow of higher reaching clasts in the upper portions of this image could possibly be masked by dense ash plumes.

On the video, the orange streaks from glowing airborne pyroclasts track to points below that suggest emission from multiple vents or an elongate vent with continuous extent, rather than a single point source, a topic returned to below in the context of an elongate trough developed on the MacKenney cone. That said, REW points out that it is hard to get a good idea of the scale from this video and that videos taken from other locations seem to show a wider, and at times two different fountain jets. Available video and photographic data has thus far prohibited estimating the width of the fountain at this stage of the eruption. REW (2013) citing Hetland (personal communication) and CONRED 856 noted that associated with these emissions the major tephra fall began, and it soon spread tens of kilometers to the N.

Early in the explosion on the 27th (exact timing unknown), a news team from a national television station (Notisiete) endured a shower of ballistics. REW (2013) noted that they were in the vicinity of Cerro Chino at probably less than 1 km from the vent, the zone with critical infrastructure most impacted (figure 53). Although most of the news team survived, reporter Anibal Archila’s death was apparently the result of direct impact from a large ballistic. His was the only icially confirmed death caused by the strong explosive phase. During a subsequent eruptive lull, a rescue team spent several dangerous hours in very close proximity to the vent, finding and rescuing missing people, and carrying out Archila’s body.

Figure 53. A truck parked directly N of the Pacaya’s active crater at Cerro Chino as seen in the aftermath of the 27-28 May explosions. Courtesy of Gustavo Chigna (INSIVUMEH).

Ballistics in excess of 0.5 m on their long axis fell at Cerro Chino and elsewhere within ~1 km of the vent area (figure 54). Some bombs on the ground reached sizes of 80 x 50 cm (Hetland personal communication) but part of that extent may have been due to splattering on impact. Farther away, the sizes of ballistics generally diminished with distance from the source. At Cerro Chino ballistic impacts broke concrete roofs, started fires in the radio shacks, and toppled antenna towers (REW, 2014; Wardman and others, 2010).

Figure 54. An example of a large bomb found in the near-source region. Courtesy of Gustavo Chigna (INSIVMEH).

When the intense phase started on the 27th, the evacuation of villages to the W (El Rodeo and El Patrocinio) was already underway. During the hours after the explosion’s onset on the 27th, more than 2,100 people were evacuated from the proximal villages to the town of San Vicente Pacaya (5 km NNW)(see related scenes in RT news, 2010 (b), table 1).

The settlements El Cedro, San Francisco de Sales, and Calderas, towns 2.5-3.5 km to the N, endured both ash as well as a dense barrage of hot ballistic bombs (figure 55). Many of the bombs were below 20 cm in diameter. Some of the ballistics pierced the corrugated (sheet metal or fiber cement) roofing common in Latin America. In some cases the ballistics also ignited fires that consumed most of the combustible contents of the buildings. Some roofs collapsed or buckled due to the load of deposited tephra.

Figure 55. Two photos taken soon after Pacaya’s 27-28 May eruptions illustrate the density of projectile penetrations through roofs of two large buildings in San Francisco de Sales (~3 km N of the MacKenney cone). Taken from REW (2013) with photo credit to Hetland.

The ballistics examined were of low density owing to vesicles larger than 1 mm in diameter. They contained sparse phenocrysts (often larger than 1 mm), most likely plagioclase (Hetland personal communication to REW and Hetland (2010).

REW (2013) noted that, from the observed damage to roofs in these villages, the density per unit area of impacts that pierced through the corrugated roofs averaged as high as on the order of 1 per square meter. Portions of the roofs in near-vent settlements also sustained many dents from bombs that delivered impacts with lower force. Although some communities were partially evacuated when many of the ballistics arrived, REW (2013) concluded that some residents remained within the communities and regions mostly affected.

Reports in Prensa Libre give insights into the scene of the evacuation and the barrage. Many of the residents evacuated on foot following narrow paths across the rugged rural terrain. Other residents remained behind in order to protect their belongings from theft. When the barrage came, those too close used whatever hard and resistant objects they could find to protect themselves, including hiding under furniture and using pots and pans to protect their heads. Some corroded metal roofs were weak prior to the eruption. Some people found refuge in buildings with heavier, concrete-slab roofs, which generally fared better.

Figure 56 shows an individual who clearly received medical attention, stitches, for a laceration on his forehead. According to REW (2013), Pacaya’s 2010 ballistic barrage caused more injuries than any recent eruptions. That said, data remain scanty on injuries rates and kinds, resultant disabilities, accident location, etc., although Wardman and others (2012) compiled some statistics.

Figure 56. Ballistic projectiles presented the most direct hazard from the 2010 explosions at Pacaya. This photo was found on the Boston Globe photo news site Boston.com (table 5). Their caption read, “A man shows the stitches he received after being injured by volcanic rock on the slopes of the Pacaya Volcano on May 28, 2010. (REUTERS/Daniel LeClair).” Courtesy of Boston.com.

The AmatitlÁn geothermal plant, located ~3 km N of the MacKenney cone to the N of San Francisco de Sales received ~20 cm of mostly lapilli-sized tephra. As Wardman and others (2012) noted, “Ballistic bombs and blocks also bombarded the plant, causing extensive damage to the plant’s roof and condenser fans. Fan blades were dented, bent and also suffered damage from abrasion. Minor denting of the intake and outlet pipe cladding was also reported however these impacts were superficial and did not require repair.” A photo showed cladding bearing multiple closely spaced dents on the side of a large pipe; the largest dent, 20 cm across, had ruptured through the sheet metal.

Post-explosion assessment of the MacKenney cone shed new light on the form and significance of the previously mentioned notch across it (a linear NW-trending trough passing through the summit, figure 57).

Figure 57. An annotated photo viewing the N side of the MacKenney cone in calm conditions at an unstated date following the May 2010 explosions. The prominent trough included a deep segment that had developed on the cone’s lower slopes (labeled ‘Possible crater’). During the 27-28 May 2010 eruption the trough appears to have served as an active fissure or series of vents emitting fountains (see figure 52 and related discussion). Courtesy of REW with photo credit to Gustavo Chigna.

The notch formed a prominent depression aligned both with the new SE-flank fissures and Cerro Chino cone on the outer NW crater rim. Portions of the RT video footage taken during vigorous stages of explosion suggests that at a paroxysmal stage of the explosion the trough served as an eruptive fissure emitting a vertically directed fountain as a curtain (table 5). REW (2013) also suggested that the eruptive fissure along the trough may have served as the vent for the ballistics that fell in previously mentioned settlements to the N.

The explosions broadest areal impact came from tephra fall. Figure 58 shows a close up of ash from a sample collected 22 km from the vent. Overall, the grain sizes ranged from sub-millimeter to centimeter size. An abundance of fine suspended particles in the air were not reported during or following the tephra fall.

Figure 58. Close up view looking at Pacaya tephra clasts collected in Guatemala City ~22 km NNE of the source. The smallest increments on ruler are in millimeters; the size range of grains here were mostly below ~ 3 mm diameter but grains under 0.2 mm were scarce to absent. The clasts consisted of black to dark brown vitric (crystal poor) scoria. Taken from REW (2013), who cited R. Cabria (personal communication).

As noted in table 1, in the afternoon on the 28th, high eruptive vigor resumed and tephra again fell on Guatemala City (CONRED 735). The ash fall on this day was lighter than on the 27th. Here aviation data (discussed below) did record the plume via satellite. The Washington Volcanic Ash Advisory Center (VAAC) noted (in their 6th advisory) an eruption in the afternoon on the 28th reaching (based on comparison of plume movement to modeling of winds aloft) ~13 km altitude.

During 29 May and onwards the intensity of volcanic activity decreased, with only relatively small eruptive plumes that occasionally produced minor tephra fall in the communities surrounding the volcano (CONRED 742). CONRED 748 noted that by the 29th, a total of 2,635 people were in shelters due to the eruption, with close to 800 home either damaged or destroyed. In the following days the attention of the emergency managers shifted from the eruption to the Tropical Storm Agatha, which had much broader extent and impact.

In the Pacaya and Guatamala City region, and along drainages carrying ash-charged run, both disasters combined. Lake AmatitlÁn rose, inundating low lying parts of the town with a water-and-ash mix (see photo documentation of impacts at Boston.com). Figure 59 is a photo taken ~12 km downstream of the Lake’s outlet.

Figure 59. The Pacaya tephra fall combined with storm run from Agnes led to swollen rivers in a ‘dual disaster.’ Those rivers formed new deposits along their beds from large amounts of in-swept debris, in this case including large boulders, trees, and a badly battered vehicle in the foreground. This press photograph was taken on 30 May 2010 as the flood water dropped. The location was the municipality of Palin, which sits along the Michatoya river downstream of Lake AmatitlÁn and ~10 km W of Pacaya. Taken from Boston.com with credit to Johan Ordonez/AFP/Getty Images.

Seismic record. INSIVUMEH and REW (2013) suggested a climax on the 27th starting shortly before 1800 local time and lasting ~40 minutes.

The seismic signal (figure 60, upper panel) contained a few scattered high amplitude events during the morning of 27 May 2010. Seismicity rose significantly about 1200 on the 27th, about doubling the RSAM values recorded during the previous 13 hours.

Figure 60. Seismicity recorded at Pacaya during the 2300 of 26 May through 1700 on 28 May (local times). The upper panel shows the seismic record and the lower panel shows the computed RSAM. Station PCG is a short-period seismometer located on Cerro Chino, ~1 km NW of Pacaya’s summit on the MacKenney cone. Courtesy of INSIVUMEH.

The first of about 10 strong peaks (seen on both the upper and lower panels of figure 60) took place around 1230 on the 27th. Those peaks represented a large escalation in seismicity an approximate doubling of the RSAM values. The highest peak on the record took place during 1730 to 1830 on the 27th, a ~6-fold increase in RSAM over the background values acquired earlier on the 27th. During the middle part of the 1800-1900 interval there was a peculiar several-minute-long period with low seismicity conspicuous on the seismic record (upper panel). After that, a series of closely spaced peaks of generally decreasing amplitude followed and then seismicity decreased substantially, particularly around 2300-2400 on the 27th. A second escalation of broadly similar size to the earlier one came on the 28th peaking at 1100 and then dropping.

In a later analysis of seismicity, Mercado and others (2012 correlated waveforms for 5 months before and 9 months after the May 2010 eruption. They noted that “No correlation was found between the events of each day during the five-month period before the eruption, thus, establishing no relationship with the periods of correlation found after the eruption. The post-eruptive sources of seismicity discovered were not active before the eruptive event of May 27, 2010, and therefore these sources must be strictly post-eruptive in nature.”

Aviation. Although there were 48 reports (Volcanic Ash Advisories, VAA’s or simply ‘advisories’) issued by the Washington Volcanic Ash Advisory Center (VAAC) on Pacaya behavior during the interval 27 May to 26 June 2010, weather clouds frequently masked the plume from the key satellite observation platform, the GOES-13 satellite. Where satellite observations of the plume were scarce or lacking, most of the VAA’s conveyed ground-based observations including media reports.

By the 3rd advisory, which was issued on the 28th, considerable ash had fallen at the International airport Aurora. There is some confusion as to the quantity of ash at the airport and over the region in general, but a photo on the 28th shows ash at the airport. Judging from ash load on the aircraft, the f walking just to the right of the aircraft, and adjacent tire tracks, the ash was on the order of ~1-cm thick (figure 61). This is in accord with INSIVUMEH’s summary report that said 5-7 mm of ash had fallen during the entire explosive 27-28 May eruption at the airport. This is also in accord with REW (2014), which discusses the complexities of assessing tephra thicknesses in more detail, and presents a preliminary isopach map that shows the S fringes of the Guatemala City urban area with 10 cm of ash and many parts of the urban area farther N, including the airport, with on the order of 1 cm of ash.

Figure 61. An American Airlines jet sits covered with ash from the Pacaya explosions at the International airport in Guatemala City on 28 May 2010. Runway cleanup took five days. The cleaning of the abrasive ash both destroyed the bituminous runway surface and all markings on it (Wardman and others, 2012). This photo was posted on the Boston Globe news website (Boston.com, see reference in table 5) with the credit to REUTERS/Daniel LeClair.

What follows is a summary of the advisories issued during 27 through 28 May (UTC).

The VAA’s frequently refer to the NAM (North American Mesoscale Model), a numerical model for short-term weather forecasting and in this case wind-velocity estimation. The model is run 4 times a day with 12 km horizontal resolution and with 1 hour temporal resolution, providing finer detail than other operational forecast models. An example of a model with less detail is the model called GFS (Global Forecast System), which predicts weather for many regions of the world, and was sometimes also used by the VAAC analysts.

The VAAC issued their 1st VAA for Pacaya during 2010 on May 27 at 1140 UTC, citing as key information sources GFS winds and INSIVUMEH. Eruption details noted small brief ash emissions near the summit at 1115 UTC. The ash cloud was not identifiable from the GOES-13 satellite owing to rain. The ash cloud was inferred to have remained low and near the volcano. GFS wind data suggested that for such a low ash cloud at that time, wind-directed transport would carry a plume S-SW and would only be significant for ~20 km. The analyst noted that eruption as then dominantly lava emission.

The 2nd advisory came out 7 hours later at 1845 UTC on the 27th indicating volcanic ash and gases to ~3.5 km altitude (noting ICAO as an information source). Ash was again not identifiable from the GOES-13 satellite owing to clouds.

The 3rd advisory, noting ‘ongoing emission of volcanic ash and gases,’ came out at 1257 UTC on the 28th, again lacking clear satellite identification of ash owing to clouds, in this case citing a thick tropical depression. This advisory relied on both a wind model (NAM winds) and an aviation meteorological report (a METAR). The advisory further noted media reports of ash on runways as discussed in the context of figure 61.

The 4th advisory was issued at 1554 UTC on the 28th, noting “increasing emissions” at 1515 UTC with INSIVUMEH reporting ash rising to 3.7 km altitude (FL 120) and spreading up to 27 km NW. Again, owing to extensive weather clouds, ash was again not visible from GOES-13 satellite.

The 5th advisory was issued at 1710 UTC on the 28th, noting “ongoing emissions” recorded at 1645 UTC. Plume has now become visible in [GOES-13] imagery and extends about 15 NMI [Nautical miles, 27 km] to the NNE of the summit. Plume top was at 3.7 km altitude (FL 120).

The 6th advisory was issued at 1915 UTC on the 28th, noting a large eruption recorded at 1815 UTC: “Large eruption seen to FL420 [42,000 feet, ~13 km altitude] based on NAM sounding for the area. Forecast winds remain mostly westerly to northwesterly. Winds at the time of observation blew the plume E at ~18 km/hr.

The 7th advisory was issued at 1930 UTC on the 28th (the last one that day); it repeated information about the eruption seen in imagery around 1815. In this advisory the wind was moving NW at 27 km/hr.

Slope stability study. Schaefer and others (2013) evaluated slope stability at Pacaya and commented on the possible implications of the trough across the MacKenney cone (figure 57). They consider the trough noted above as an example of a recent, smaller-volume collapse.

Specifically, they studied the SW flank of the edifice and developed a geomechanical model based upon field observations and laboratory tests of intact rocks from Pacaya. Their study included analysis of slope stability using numerical techniques and consideration of forces from gravity, magmatic pressure, and seismic loading as triggering mechanisms for slope failure.

Given the cone’s structural and seismo-tectonic setting, the likely magma pressures, and the history of past behavior, they suggested Pacaya lacked substantial gravitational stability.

INSIVUMEH reported a gradual increase of tremor amplitude at Pacaya during 17-18 June. Observers noted that small ash ejections from Mackenney cone were dispersed around the crater. Tremor continued to be detected during 20-22 June. Ash emissions continued to be confined to the crater area during 21-22 June, and incandescence from the crater was visible at night.

INSIVUMEH reported that during 7-8 June white and blue fumarolic plumes rose above Pacaya's Mackenney cone. Ash emissions were observed about every 3-4 hours, and the seismic network detected signals indicating collapsed within the crater along with ash emissions.

INSIVUMEH reported that during 12-13 February a series of weak explosions from Pacaya's Mackenney Crater generated dark gray ash plumes that rose 500-700 m above the crater and, along with fumarolic plumes, drifted 1.5 km S. During 14-15 February weak explosions continued to generate ash plumes; ash and fumarolic plumes drifted 800 m SE. The next day fumarolic and ash plumes drifted S and SW at a low altitude. During 16-17 February fumarolic plumes with small amounts of ash rose 100 m and drifted E.

In a special notice INSIVUMEH reported that on 28 January ash emissions originating from Pacaya's Mackenney Crater drifted 4 km S and SW. During field observations, scientists saw a defined central crater, 40-50 m in diameter, and ash emissions. Gas plumes rose from an area on the S flank. Seismic data was characterized by tremor and low-frequency events. In a report from 1 February, INSIVUMEH stated that low-altitude water vapor plumes with minor amounts of ash drifted W and SW. During 1-2 February fumarolic plumes rose 50 m and drifted 600 m S.

INSIVUMEH reported that on most days during 30 June-13 July, fumarolic plumes rose above Pacaya and drifted up to 1 km W, SW, and S. Associated seismicity was notable on 4, 6, 9-11, and 13 July. Elevated seismicity on 10 July corresponded to minor explosions from Mackenney Crater.

INSIVUMEH reported that during 9-10 April an explosion from Pacaya generated an ash plume that rose 50 m and drifted 1 km SSE. During 9-11 and 13-15 April white plumes from Mackenney Crater drifted S and N.

INSIVUMEH reported that during 16-20 January gas plumes rose from Pacaya and a lava flow on the S flank remained active. A report on 21 January noted that the S-flank lava flow was 3.6 km long and continued to slowly advance, burning vegetation between the Rodeo and Los Pocitos roads. Volcanologists observed that the cone in Mackenney Crater had been completely destroyed, leaving a deep crater that produced fumarolic activity.

INSIVUMEH reported that during 10-11 September explosions at Pacaya generated diffuse ash plumes that rose 150 m and drifted 150 m NW. There were no direct visual observations during 11-12 September, though the seismic network recorded tremor and small explosions. Explosions during 12-13 September ejected material 50-100 m high. White and blue plumes rose 300 m and drifted N. In a special notice on 13 September INSIVUMEH noted that the cone in MacKenney Crater had already grown above the crater rim; incandescence from the cone was visible from many areas around the volcano, including the capital.

Weak-to-moderate explosions generated rumbling during 14-15 September. An increased number of explosions were detected during 15-16 September. Material was ejected above the crater and ash plumes drifted 3 km W and SW. The seismic network recorded tremor and explosions that occurred about every 5 minutes.

INSIVUMEH reported that on 26 August Strombolian explosions from Pacaya's MacKenney Crater ejected incandescent material 50 m above the crater. Seismicity remained high. During 26-27 August explosions were detected at intervals between 10 seconds and 4 minutes. Incandescent material was ejected 75 m high. A lava flow 150-200 m long was active on the W flank.

INSIVUMEH reported that during 13-14 August the seismic network at Pacaya recorded weak tremor and explosions, although no ash plumes were observed. Incandescence from the crater was visible at night during 14-15 August. White and blue plumes rose from the crater on 15 August. A Strombolian eruption occurred on 16 August from 1915 to 2245, producing a 300-m-long lava flow that traveled W from MacKenney Crater. The seismic network recorded a few gas explosions and intermittent tremor during 17-18 August. Seismicity increased on 19 August; tremor and explosions were detected. On 20 August white plumes rose to low heights and drifted N.

INSIVUMEH reported that during 7-8 August white vapor plumes rose 200 m above Pacaya and drifted E. On 9 August seismicity increased, and Strombolian explosions ejected tephra 200 m above MacKenney Crater and onto the flanks, 400 m away from crater. The next day the number and magnitudes of explosions increased, and seismic signals indicating fluid movement were recorded. Tephra was again ejected 400 m away from MacKenney Crater, causing small avalanches of volcanic material on the flanks. On 12 August fumarolic plumes rose 50 m. Cloud cover prevented observations of the crater on 13 August; however, the seismic network recorded a few gas explosions and tremor.

In a special bulletin on 24 July, INSIVUMEH noted that the eruption at Pacaya had been changing during the previous few days, especially the seismic pattern. Seismic signals indicating explosions and ejections of material lasted up to seven minutes; the events were low frequency and long duration. The cone continued to grow and was 30 m high earlier in the week. By 24 July the cone was 4 m above the MacKenney crater rim. Seismicity again increased. On 25 July weak explosions and incandescence from the cone were observed at night. Rumbling was heard. On 29 July incandescence from the crater was observed for a few hours in the morning, and a plume rose at most 100 m and drifted S. An eruption on 30 July included a high-energy phase that lasted for four hours and incandescent material that was ejected 250 m above the cone. A diffuse ash plume drifted 2 km N, causing ashfall in areas downwind, and another ash plume drifted 5 km S. Activity then declined considerably; explosions were not observed and seismicity decreased, although signals indicating fluid movement continued to be detected.

INSIVUMEH reported that blue-colored emissions from Pacaya were visible drifting SW and W at low altitudes on 26 June. Strombolian activity was observed from MacKenney cone the following day; weak-to-moderate explosions ejected small amounts of tephra 8 m above the crater that were then deposited on the W flank. Audible explosions were noted up to 5 km away. Incandescence was visible at night on 27 June. White fumarolic plumes rose 300 m above the cone on 28 and 30 June; white and blue fumarolic plumes drifted SW during 1-2 July. A recent investigation of MacKenney cone determined that a 15 m high cone had been the source of recent explosive activity.

INSIVUMEH reported that incandescence from Pacaya's crater was observed late at night on 28 May. Weather conditions prevented observations the next day. On 30 May a small effusive eruption occurred for about two hours. A small explosion ejected ash and lapilli 200 m above the crater that was then deposited within 400 m of the crater. Inclement weather prevented observations the rest of the day; however, the seismic network detected tremor and weak explosions.

INSIVUMEH reported that during 22-23 May weak Strombolian activity at Pacaya's MacKenney cone was detected by the seismic network. On 24 May white plumes rose 600 m and drifted S. In a special bulletin on 25 May, INSIVUMEH noted that the eruptive pattern had changed during the previous few days. Explosions were more continuous and energetic, and were detected 3-5 minutes apart. Explosions ejected bombs and generated rumbles heard 4 km away. Cloud cover mostly prevented views on 27 May, but blue gas plumes were observed. Occasional weak glow from the crater was observed on 28 May.

INSIVUMEH reported that weak incandescence from Pacaya's MacKenney cone was observed through the night during 15-16 May. Blue and white plumes rose 800 m and drifted S. On 17 May white plumes drifted W and NW. Incandescence from the crater was again observed at night during 19-21 May. On 20 and 21 May Strombolian activity ejected material 25 m above the crater.

INSIVUMEH reported that on 23 April fumarolic plumes from Pacaya's MacKenney cone rose 100 m and drifted N. On 24 April tephra was ejected 25 m high by weak explosions. Incandescence from the crater was observed through the night, and explosions were detected the next day. Incandescence and explosions were again detected on 29 April.

Based on INSIVUMEH notices, CONRED reported that explosions at Pacaya detected on 13 February were accompanied by rumbling. No material was ejected. The next day a diffuse white plume rose 200 m and drifted W and SW. Rumbling was heard in San Francisco de Sales (5 km N) and San Vicente Pacaya (5 km NW).

INSIVUMEH noted in a special report that on 28 December patterns of activity from Pacaya's MacKenney cone changed; three explosions detected at 1150 generated plumes that rose less than 500 m and drifted 5 km W and SW. During 30 December-1 January bluish-white plumes rose 50 m and drifted S and SW.

INSIVUMEH noted in a special report that on 20 March patterns of seismicity and emissions from Pacaya's MacKenney cone changed, although remained characteristic of normal behavior for the volcano. On 26 March avalanches were detected and during 26-27 March gas plumes drifted S and SW. The report noted that after the eruption on 27 May 2010 only fumarolic emissions, mainly composed of water vapor, rose from MacKenney cone.

INSIVUMEH reported on 20 January that a blue plume rose from the base of the NW flank of Pacaya's MacKenney cone. The Pacaya National Park authority was advised to not allow tourists near the area with the new plume. During 21-24 January fumarolic activity in the crater had variable intensity.

INSIVUMEH reported that on 20 July Strombolian explosions from Pacaya's MacKenney cone ejected ash that fell in neighboring areas. During 20-21 July there were 90 explosions recorded by the seismic network. Based on information from INSIVUMEH, the Washington VAAC reported that on 22 July a plume rose to an altitude of 4.6 km (15,000 ft) a.s.l. and drifted N. A weak thermal anomaly was seen in subsequent images. The next day, ash plumes drifted N at an altitude of 4.1 km (13,500 ft) a.s.l. and produced ashfall in areas within 10 km. On 25 July, INSIVUMEH noted that Strombolian explosions ejected tephra 100 m above the crater, and generated ash plumes that rose 300 m above the crater and drifted 10 km SW. Ejected blocks fell onto the flanks.

On 7 and 9 July, INSIVUMEH reported that white plumes rose from Pacaya's MacKenney cone and drifted N. Small explosions were detected by the seismic network on 7 July. According to a CONRED notice, INSIVUMEH reported that an explosion on 13 July generated an ash plume that rose 300 m above the crater and drifted SW. Ash and tephra fell in nearby areas, and 150 people were evacuated.

INSIVUMEH reported that during 9-10 June Pacaya's MacKenney cone emitted white-and-blue fumarolic plumes that rose 300 m high, and generated sounds audible up to 5 km away that resembled airplane engines. Occasional ash plumes drifted 2 km NW. Lava flows continued to be active on the SE flank and moved at a speed of about 1 m per hour. Explosions continued from a lateral crater.

INSIVUMEH reported that on 3 June Strombolian activity from Pacaya ejected material 200 m into the air. During 5-6 June no explosions or ash emissions were noted, and seismic energy remained stable. Bluish-white plumes rose 700 m and drifted W. On 7 June an explosion ejected ash 100 m above the crater resulting in an ash plume that drifted 2 km NW. Blue-and-white plumes continued to rise from MacKenney cone. Multiple lava flows remained active and had traveled as far as 3.5 km by 6 June.

A Strombolian eruption from Pacaya's MacKenney cone that began on 27 May was characterized in a report from CONRED as having constant explosions that ejected material 500 m into the air. Ash plumes rose 1.5 km above the crater and drifted W and SW, causing ashfall in multiple areas. The community of El Patrocinio (about 5 km W) evacuated and residents in El Rodeo (4 km WSW) were ordered to evacuate. Due to extensive tephra fall, authorities recommended that residents clean off ash from their roofs and refrain from driving.

INSIVUMEH reported a continuing series of explosions 5-10 seconds apart that ejected black ash up to 1 km above the crater on 28 May. Seismic signals reflected the explosions in addition to tremor. Ash plumes drifted 20-30 km NW, causing ashfall in areas downwind, including in Guatemala City, about 30 km NNE. CONRED reported a short time later that about 1,600 people had been evacuated from six towns 3-4 km W, NNW, N, and NNE, and that Aurora International Airport was closed. According to a map posted by CONRED, blocks fell in areas as far away as 12 km NE and ash was reported in areas E of Chinautla, 37 km NNE. News media reported that one person (a reporter) died and three children were missing.

On 29 May, a 90-m-wide lava flow that traveled SSE at an estimated rate of 100 m per hour burned three houses on the Pacaya Grande ranch. The lava was within 450 m of some other properties including El Chupadero, located 2-2.5 km S of the crater, and disrupted an access road from El Caracol (3 km SW) and Los Pocitos (5.5 km S). Explosions ejected ash 300-500 m above the crater. INSIVUMEH reported on 1 June that the Strombolian activity continued. Explosions ejected ash as high as 700 m above the crater and ash plumes drifted NW. Two lava flows were seen traveling SW and SE.

On 20 May, INSIVUMEH reported that small explosions and incandescence from Pacaya's MacKenney cone were accompanied by white and blue plumes. Multiple lava flows traveled as far as 1.6 km down the SW flank.

On 10 February, INSIVUMEH reported that lava flows from Pacaya, descending the flanks since April 2006, continued to flow down the E flank onto part of the meseta. During 11-16 February, lava flows 100-400 m long descended the E and NE flanks. Avalanches of blocks from the lava-flow fronts set fire to local vegetation.

INSIVUMEH reported that activity from Pacaya consisting of effusion of lava flows, the source of which had migrated towards the S from the N flank since April 2006, ceased on 30 January 2010. On 5 February, Strombolian explosions from MacKenney cone ejected material 30 m into the air and lava from the crater moved down the flank. The activity was heard in the village of San Francisco de Sales, 5 km N. A new lava flow originating from a depression on the NE flank was seen on 6 February.

On 8, 11, and 12 January, INSIVUMEH reported that white and blue fumarolic plumes from Pacaya's MacKenney cone rose up 400 m and drifted S and SW. Multiple lava flows on the S, SW, and W flanks traveled 25-200 m. Incandescence was noted at night from one of the inter-crater cones on 8 January and from MacKenney cone on 11 and 12 January.

On 5, 8, and 9 June, INSIVUMEH reported that fumarolic plumes from Pacaya's MacKenney cone rose 50-200 m and drifted W and SW. During the reporting period, two to four lava flows, each 150-300 m long, were emitted from an area on the lower S flank, SW from the main edifice.

INSIVUMEH reported that explosions in March ejected greater amounts of material that was deposited in the crater, enlarging the cones there. On 23 March, visual and audible changes in Strombolian activity were noted. Vigorous degassing produced sounds resembled airplane engines. In a report issued on 3 April, INSIVUMEH stated that Strombolian explosions from MacKenney cone during the previous few days ejected material 25 m into the air. On 2 April, lava flow volume increased, sending four lava flows W and one SW; the flows traveled 25-200 m. The seismic network detected tremor and explosions. On 6 April, lava flows on the W flank traveled 150-300 m, causing lava to pile up on the SW flank. Activity from MacKenney cone was continuous; one cone emitted gas and explosions about every 5-10 minutes, and a second cone ejected tephra 25 m high. On 7 April, one lava flow traveled 150 m W and one traveled 200 m SW. INSIVUMEH recommended that CONRED coordinate with authorities in Pacaya National Park to restrict visitors from climbing Pacaya.

INSIVUMEH reported that on 30 January and 3 February white and blue fumarolic plumes from Pacaya's MacKenney cone drifted S and SW at a low altitude. One lava flow, 75-100 m long, traveled down the SW flank.

On 12 December INSIVUMEH reported that fumarolic plumes from Pacaya's MacKenney cone drifted NE at a low altitude. Three lava flows, 150, 250, and 800 m long, were observed from the S. Seismic data indicated small explosions at the crater.

Based on analysis of satellite imagery, the Washington VAAC reported that on 2 November a possible ash-and-gas plume was emitted from Pacaya and drifted E. On 3 November, INSIVUMEH reported that fumarolic plumes drifted S at a low altitude. Ash occasionally entrained by strong winds drifted S. Multiple lava flows on the S and SW flanks of MacKenney cone traveled a maximum distance of 400 m on 3 and 4 November, and continued to fill in the area between the cone and Cerro Chino crater to the N. Fumarolic plumes drifted E on 4 November.

INSIVUMEH reported that, during 8-14 October, multiple lava flows on the W and SW flanks of Pacaya's MacKenney cone traveled a maximum distance of 250 m and continued to fill in the area between the cone and Cerro Chino crater to the N. Avalanches occurred from the lava-flow fronts on 8 October. Fumarolic plumes drifted SW.

INSIVUMEH reported that during 21-26 August, fumarolic plumes from Pacaya's MacKenney cone rose to an altitude of 3.2 km (10,500 ft) a.s.l. and drifted W and SW. Incandescence from the crater was occasionally seen at night. Lava flows on the SW flank branched and traveled a maximum of 300 m; lava continued to fill in the area between MacKenney cone and Cerro Chino crater to the N. Avalanches occurred from the lava-flow fronts on 26 August.

INSIVUMEH reported that during 9-16 July, Strombolian activity from Pacaya's MacKenney cone was mainly characterized by explosions approximately 2-3 minutes apart. Pyroclastic material was ejected about 25 m above the crater. Lava flowed 100-200 m down the NW flank and continued to slowly fill in the area between MacKenney cone and Cerro Chino crater to the N. On 16 July, fumarolic plumes drifted SW.

During 18-24 June, INSIVUMEH reported that white fumarolic plumes from Pacaya's MacKenney cone drifted S and W. Lava flows traveled 50-100 m NW in the area between MacKenney cone and Cerro Chino crater to the N. Incandescence in the crater was observed at night.

INSIVUMEH reported that during 19-20 May white fumarolic plumes from Pacaya's MacKenney cone drifted W. Lava flows from the base of the NW flank traveled 100 m NW in the area between MacKenney cone and Cerro Chino crater to the N. The seismic network recorded small explosions and occasional tremor.

INSIVUMEH reported that during 6-19 February white and blue fumarolic plumes from Pacaya's MacKenney cone rose to altitudes of 2.6-2.7 km (8,500-8,900 ft) a.s.l. and drifted S, SW, and W. About two to five lava flows per day traveled about 50-200 m to the W and NW, slowly filling in the area between MacKenney cone and Cerro Chino crater to the N. Explosions on 8 February propelled fragments 100 m above the summit.

INSIVUMEH reported on 24 January that white and blue fumarolic plumes from Pacaya's MacKenney cone drifted S and SW. Four lava flows traveled about 100 m to the W. Based on reports from INSIVUMEH, CONRED reported on 28 January that the Alert Level was lowered to Green.

INSIVUMEH reported on 11 January that continuous effusion of lava on Pacaya's W flank resulted in a 150-200 m-long lava flow. Fumaroles produced white and blue plumes that drifted S, and incandescence at night was observed from the summit. CONRED reiterated that the Alert Level remained at Yellow.

According to CONRED, a 17 December INSIVUMEH report noted changes in Pacaya's behavior. On 19 December, CONRED issued a bulletin noting that INSIVUMEH reported observations of white and blue "smoke" plumes. The plumes rose to an altitude of 2.8 km (9,200 ft) a.s.l. and drifted S. Lava flows were unchanged from previous days. Based on these reports, CONRED raised the Alert Level to Yellow in surrounding communities.

INSIVUMEH reported that during 12-17 December constant lava flows on the W flank of Pacaya's MacKenney cone were about 100-200 m in length. Based on seismic interpretation, a small explosion occurred in the crater on 12 December. Fumaroles produced plumes that drifted S and SW and incandescence at night was observed from the summit.

On 7 September, INSIVUMEH reported that lava flows on the W flank of Pacaya's MacKenney cone were about 150 m in length. During the week prior, fumaroles in the crater produced white and blue plumes that drifted S and SW. Incandescence was observed from the summit.

On 13 July, INSIVUMEH reported that lava on the NE flank of Pacaya's MacKenney Cone branched into three flows. Two flows traveled 100 m and 200 m to the N and one traveled 300 m to the NE. Incandescence was observed from a hornito in the summit crater. White plumes were observed.

INSIVUMEH reported that on 31 May, and 1 and 5 June, lava from the NE flank of Pacaya's MacKenney Cone continued to flow and pooled between the base of the volcano and the plateau. Gas plumes from the summit drifted N and SW and rose to altitudes of 2.8 km (9,200 ft) a.s.l. on 5 June. Small pyroclastic explosions from the N part of MacKenney Cone were occasionally observed.

INSIVUMEH reported that on 9 and 12 March, fumurolic activity from Pacaya's MacKenney Cone produced plumes that rose to 3 km (9,800 ft) a.s.l. and drifted S and SW. Incandescence was reflected in the plumes on 9 March. A lava flow from a crater on the NE flank was visible on 12 and 13 March to distances of 300 m.

INSIVUMEH reported that during 4-5 January, Strombolian eruptions from Pacaya produced incandescent material that was expelled at 2-40-second intervals, up to 100 m above the crater. Gas clouds reached an altitude of 2.7 km (8,900 ft) a.s.l. and drifted S. A lava flow 50 m in length pooled near the NE edge of MacKenney Cone.

INSIVUMEH reported that on 15 and 17 November lava flows from Pacaya traveled about 100-150 m N and NE. On 17 November, fumarolic emissions produced a white cloud that surrounded the S flank of MacKenney Cone.

Lava flows have continued at Pacaya during 14-29 September, as reported by INSIVUMEH. The flows slowly advanced W towards Cerro Chino and NE towards Cerro Grande. White fumarolic emissions continued to rise from the MacKenney Cone.

The Washington VAAC reported that emissions from Pacaya with possible ash content were visible on satellite imagery on 28 August. The plumes reached altitudes of ~3 km (~10,000 ft) a.s.l. and drifted W. A thermal anomaly was identified on the imagery.

A white- and blue-colored "smoke" plume from Pacaya was observed on 5 and 7 July and drifted NW and W, and a white gas cloud on 10 July reached 800 m above the crater (11,000 ft a.s.l.) and drifted SW. Lava flows toward the NW reached lengths of 800 m. Incandescence was observed on 7 July.

During 22-28 March, incandescent volcanic material was ejected tens of meters above Pacaya and lava flows extended ~100 m down the volcano's S flank. On the 28th, a new lava flow was emitted from the SW edge of the active crater. The flow reached ~150 m and avalanches occurred from the lava-flow fronts.

Strombolian explosions at Pacaya on 9 March occurred at intervals of 10-30 seconds and threw volcanic material 50-100 m above the volcano. On the evening of 12 March, there was an increase in Strombolian activity, with material reaching 150-250 m above Mackenney Cone.

During 7-11 September, occasional Strombolian activity occurred at Pacaya. Volcanic bombs were emitted from two craters, reaching up to 30 m above their rims. Incandescence from lava flows on the volcano's SE flank was visible on several nights.

By 27 June a lava flow extended ~300 m down the SW flank. A white column reached ~150 m (8,860 ft a.s.l.) over the central crater and extended SW. Incandescent lava expulsions reached a height of 15-50 m. On the night of 27 June two rivers of lava were observed in front of the Chinese hill, and were 75 and 150 m long. A constant expulsion of pyroclastic material was observed to reach 20-30 m above the crater.

Lava emission continued at Pacaya during 4-9 May. Earlier in the period, on 4 May, three lava flows were active, extending up to 100 m down the SW flank and 150 m W in the direction of Cerro Chino. On 9 May two flows from the base of the intracrater cone were active, reaching 200 m down the W flank. Plumes from the MacKenney Cone rose as high as 800 m above the crater.

INSIVUMEH reported that activity at Pacaya during 14-18 October had not significantly changed in 2 months. Weak glow was visible from MacKenney cone, variable tremor was recorded, and white plumes rose to ~200 m above the crater.

On 5 July at 0715 a very thin ash and/or gas plume was visible at Pacaya on satellite imagery at a height of ~3 km a.s.l. extending ~7.5 km SW. By 1430 the plume was no longer visible, possibly obscured by thunderstorm clouds in the area, and the Guatemala Volcano Observatory reported that only steam was emitted from Pacaya.

The Washington VAAC reported that on 17 June at 0815 satellite imagery showed a very thin SW-drifting plume of unknown composition emitted from Pacaya. In addition, satellite imagery showed a faint hotspot at the volcano's summit.

The agency responsible for monitoring active volcanoes in Guatemala, INSIVUMEH, observed several indications of renewed eruptive vigor at Pacaya. A local seismometer recorded over 700 earthquakes per day in comparison to 100 earthquakes per day recorded approximately 2 weeks earlier. SO2 gas emissions increased from 253 tons/day to 550 tons/day. In addition, lava was visible in the summit region at MacKenny Crater.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

January 1970 (CSLP 03-70)

Elevated activity in late 1969; strong eruption on 7 January

Card 0843 (8 January 1970) Elevated activity in late 1969; strong eruption on 7 January

The following cable was received from Guatemala City on 9 January 1970. The Pacaya volcano began a strong eruption last night and caused unrest in a vast zone. Long tongues of fire rose into the darkness and the light gave the impression of a huge fire. The volcano has frequently been the epicenter of strong seismic movements.

The following report is from Richard Stoiber. The magnitude of the eruption . . . has been increasing for the past several months. On 15 December, I observed a regular eruption which included ejecta, bombs, and scoria coming out of MacKenney Cone, and reaching a height above the summit. No ash was observed.

February 1972 (CSLP 11-72)

Lava flow from a new vent on the NW flank

Card 1356 (17 February 1972) Lava flow from a new vent on the NW flank

The following report was received by R. Stoiber from Sam Bonis, Instituto Geografico Nacional, Guatemala, on 16 February. The report was the result of observations by Sam Bonis, Tom Loucks of Dartmouth College, and of Alexander Ritchie of the University of Texas. "2 February a lava flow started from at least one of the three new vents on the south flank of Cerro Chino cinder cone forming part of the Volcan Pacaya complex lying on the NW flank of the currently active Volcan Pacaya. 4 February it had traveled ~1.5 km in a westerly direction. At this time it was flowing from only the lowest vent at an elevation of about 2,500 m. By 11 February the volume of the flow and advance had greatly diminished. An ash eruption, probably from the main Pacaya cone, accompanied the earlier stages of the lava flow and covered the Pacaya area to a depth of 2 cm."

December 1972 (CSLP 11-72)

Lava flow from SW flank still active

Card 1519 (27 December 1972) Lava flow from SW flank still active

Volcano Pacaya has renewed eruptive activity. The present phase started with ash eruptions from MacKenney Cone on or about 15 October 1972. This was followed by a lava flow issuing from the lower southwest flank and still continues. The flow has reached a length of about 1.5 km. Observations by teams of Instituto Geografico and Dartmouth geologists continue.

The present period of activity was initiated on 2 February 1972 with lava flows from new vents and fissures on the northwestern flank of the principal Pacaya Cone as well as from lower vents on the south flank of the adjacent Cerro Chino, a subsidiary sister cone. This activity continued for about a month and produced a lava flow approximately 1.5 km long. Slow collapse engulfment and resulting enlargement of the active McKenny Crater of Pacaya became noticeable in early September, leading to the current activity. All flows and ashes have been of the basaltic composition.

January 1973 (CSLP 08-73)

Ash eruptions and lava emission

Card 1539 (19 January 1973) Ash eruptions and lava emission

"Volcano Pacaya resumed ash eruptions from MacKenney Cone 13 January 1973. Eruptions occurred at 4-5 minute intervals. Lava continued to issue from the vent on the lower southwest flank but increased in amount concurrently with the ash eruption phase, from approximately 20 m3 per minute (12 January) to approximately 60 m3 per minute. At 1330 lava started issuing from a formerly active vent approximately 15 m above the lower vent at a rate of 96 m3 per minute.

Summit activity was not apparent on 14 January but lava continued to issue from both vents at approximately the same rate as the previous day. The flows continue to be olivine basalt. Pacaya has been issuing lava since mid-October 1972. A preliminary ground-based infrared thermal survey revealed no anomalously hot areas other than those associated with present and recent vents.

"Pacaya, inactive since the October 1975 lava flow, resumed activity for at least 1 day prior to the 4 February earthquake. Black steam clouds from MacKenney Crater were seen in Guatemala City. A group camped at Pacaya on the morning of the 4 February earthquake reported that a black steam cloud was discharged from MacKenney Crater immediately after the event. Eruptions continued until sunrise. Primarily ash and steam with very few small blocks were discharged. At sunrise the eruptions ceased and Pacaya remained quiet until sunset when similar activity resumed. A group from Dartmouth College and IGN visited Pacaya on 16 February. There were no eruptions, but earth tremors of several seconds duration were felt during the 18 hours that the party was on the summit. Gas condensate samples were collected from a fumarole with a temperature of 138°C on the rim of MacKenney Crater.

"The summit crater of Pacaya was cut by at least seven steaming fractures that described a radial pattern. These fractures had been noted on visits to the summit in November 1975 and January 1976. A thermal infrared map was made of the W flank of Pacaya on the early morning of 18 February from a station near El Patrocinio. A large zone approximately 100 m square, at an elevation of 2,250 m, contained apparent surface temperatures 10° above ambient with a maximum apparent surface temperature of 40° above ambient. This zone bears watching as the possible locus of a future lava flow. An earth tremor of 10 seconds duration was felt while making the thermal map at 0620. At 0903 a dark cloud was erupted from MacKenney Crater. Six more similar eruptions took place in the next hour before the summit was obscured by clouds. Despite the absence of a favorable wind direction, ash from the eruptions fell near El Patrocinio, 2,200 m from the vent. The eruption clouds rose approximately 500 m above the summit and rocks were heard cascading down the slopes."

After a month of increasing steam emission, a 1-day series of ash eruptions occurred on 27 August from MacKenney Crater, the active vent of the 1965-75 eruption. Although the eruptions were conspicuous, the amount of ash ejected was not large, and was carried W by the wind. Small eruptions were also reported 19 August (black ash) and 20 August, accompanied by a strong sulfur odor near the volcano.

Pacaya displayed weak Strombolian activity during a visit by Michigan Tech. Univ. geologists on 14 February, the first Strombolian activity observed at Pacaya since 1975. Gas emissions have characterized the activity since late 1977.

Lava was fountaining to 200 m at 10-second to 1-minute intervals from two coalesced spatter vents in the center of MacKenney Crater, high on the WNW flank. Four subsidiary vents, two N of the spatter vents and two W of them (in the direction of the volcano summit), also ejected lava. New pahoehoe lava flows, some of which were moving, had filled the N half of the crater floor to the rim. The fountaining was interspersed with intense, pulsating gas emission from the spatter vents.

By 20 February, when Robert Hodder climbed Pacaya, one lava flow had traveled a quarter of the way (about 200 m) down the N flank of MacKenney Crater cone, over one of the September 1969 flows. Within the crater, cracks and pressure ridges in the lava crust indicated continued lava movement. Strombolian activity was occurring at about 30-minute intervals. Patches of sublimate were visible on the SE crater wall.

During a second climb on 28 February Hodder observed that aa lava had flowed about 750 m from the crater rim to the base of MacKenney Crater cone, into the trough between it and the rim of the older Pacaya edifice. The level of lava in the crater had risen. The 2 vents observed on 14 February had totally coalesced and had built cones about 15 m high. The lava crust seemed solid, but incandescence showed through surface cracks at night. Strombolian activity occurred about every 20 minutes. Large cow-dung bombs, hurled as high as 100 m, fell onto the cones and the lava crust. Bomb ejection was sometimes preceded by a puffy steam cloud at least 300 m high. Sublimate solidly coated the SE crater wall. Hodder noted that this eruption seemed similar to that of 1969.

Newspapers reported that vigorous magmatic activity had begun at Pacaya on 9 February. According to newspapers, activity peaked 18 February at 1730 as lava overflowed the N rim of the crater and began to move down Pacaya's N flank.

Michigan Tech. Univ. geologists climbed Pacaya again on 5 March. Since their last visit 14 February, the level of the lava lake in MacKenney Crater had risen considerably, the two coalesced spatter vents had grown, and an additional small spatter vent had formed in the S part of the crater on the lava lake surface. The new S vent continuously extruded two pahoehoe lava flows but was not the source of any Strombolian explosions. A few small pahoehoe flows were also moving across the E side of the crusted lava lake surface. Nearly continuous weak Strombolian activity occurred from the two older vents. The smaller N vent had many small Strombolian explosions at intervals of 10-20 seconds. From the larger vent, activity was cyclical, consisting of a 1-5-second explosion that ejected spatter to 200-300 m above the vent, followed almost instantly by a large increase in gas emission that peaked in about 1 minute, decreased slowly, then dropped sharply about 30 seconds before the next explosion. Gases above the vent had an intense blue color. The alignment of the three vents in MacKenney Crater indicated that the activity may have been from a fissure trending approximately N-S.

The geologists estimated that the lava flow descending the N flank had a volume of about 2 x 104 m3 on 5 March. They estimated the total volume of 1981 lava at about 1 x 106 m3, for an average eruption rate of about 4 x 104 m3/day. The lava was petrologically similar to lavas from eruptions since 1970, consisting of basalt with abundant plagioclase phenocrysts and sparse olivine phenocrysts.

Geologists climbed to Pacaya's summit on 12 February. During their observations, lava from a 10-m-high hornito at about 2,100 m altitude on the SW flank flowed into a lava tube that extended about 0.5 km downslope. Lava emerged from the end of the tube and continued several kilometers farther down the SSW flank, forming an aa flow about 10 m wide, incandescent in daylight, that was bordered by levees. Two smaller subflows branched from the main flow 0.5-1 km below the end of the lava tube. Lava flowing through the tube appeared to be of pahoehoe type when viewed through a skylight just upslope from its distal end. A considerable quantity of blue fume escaped through the skylight. Near the active hornito, two smaller ones, each a few meters high, were no longer emitting lava. Loud booming from the area of the hornitos was heard every few seconds. According to a security guard working on Pacaya, activity from the hornitos had begun about 3 weeks earlier.

Since geologists visited Pacaya in March 1981, several additional lava flows had spilled over the rim and moved down the flank. Residents of the area said that lava effusion had stopped a few months prior to the February 1982 visit but Strombolian explosions had continued until activity began at the SW flank hornitos in early 1982. In February 1982, MacKenney Crater contained a single spatter cone, elongate NE-SW, 50-60 m in diameter, and 30-35 m high, breached on the NE side by a fissure that was vigorously emitting vapor. Lava had apparently flowed through this breach in the past. No incandescent rock was visible within the spatter cone, which had a crater no more than 50 m deep. Considerable sublimate deposition had occurred on the spatter cone's NW side. Intense booming from within the spatter cone occurred every 10-15 seconds, occasionally causing felt shaking on parts of the cone for a few seconds, but was not accompanied by any obvious increase in vapor emission.

Maurice Krafft climbed Pacaya 28-29 January and observed lava emerging sluggishly from the SW flank vent that had been much more vigorously active in early 1982. The lava formed a single flow, about 3-4 m thick and less than 50 m long, that advanced only about 1 m/day. When Alfredo MacKenney had climbed the volcano 3 weeks earlier, the same vent had been feeding 3 aa flows, each about 2-3 m wide and 20 m long. None of these tiny flows were still active in late January. Moderate degassing continued from summit vents.

In March 1983, small Strombolian explosions began in MacKenney Crater, 50 m in diameter and 30 m deep. Similar activity continued through June. Activity increased on 17 July when explosions were heard and glow seen from nearby towns. By 31 July, the crater was completely filled with blocks and ash. A pyramidal cone 30 m in diameter and 20 m high, with a small upper crater, was present within the summit crater on 21 August. On 4 September, activity increased again as lava flowed from a hornito on the upper S flank of the newly formed cone. On 11 September, the cone was 25 m high and 50 m in diameter with three active vents trending N and two lava flows emerging from its W flank. The two upper vents produced continuous explosions on 15 September, and lava flowed about 200 m downslope from a hornito on the N flank of the summit crater.

Geologists from Michigan Tech. Univ. and the Instituto Geográfico Militar (IGM) visited the volcano 13 and 16 November. Cloudy weather on the 13th allowed only brief glimpses of the crater; activity appeared to be weak but some spattering may have been occurring. On 16 November, the geologists noted that the active cone, centered on the S edge of MacKenney Crater, had grown to more than 100 m in height and filled 1/3-1/2 of the crater. Several Strombolian bursts per minute occurred from a vent at the top of the cone, ejecting spatter to 20 m height. Bombs reached 30 cm in maximum dimension. Aa lava emerged from below the base of the active cone, flowed down the crater's NW flank, and was beginning to pond in the saddle between MacKenney Crater and an older cone (Cerro Chino).

During December 1983 and the next 2 months, activity increased and the new cone grew in height and diameter. Many lava flows emerged from below its N base. In March, two lava flows were extruded from the cone's S base while activity declined in its upper crater. On 15 May, people living near the volcano heard strong explosive activity. When the volcano was visited on 20 May, half of the active cone had been destroyed. Small pyroclastic explosions were occurring and a great quantity of vapor was being emitted. On 10 June, several lava flows were emerging in the saddle between the active crater and Cerro Chino (figure 1). In July, lava continued to fill this area and flowed W. On 5 August, Strombolian activity had increased in the upper crater. In August and early September lava emerged slightly SE of the June-July vents. On 2 September, minor pyroclastic activity was occurring from MacKenney Crater.

Lava emission in saddle area stops; lava flow and tephra from small cone in MacKenney Crater

Lava flows from the saddle vent that began erupting on 5 August 1984 remained active until 6 February 1985 and built a cone approximately 50 m high.

On 1 January, minor pyroclastic activity, which could only be seen from the crater rim, resumed at MacKenney Crater. By 10 February, a steep-sided lava cone about 6 m high had developed within the crater. Pyroclastic material was ejected from the top of the small cone, a lava flow emerged from its E flank, and a lava spine had grown on its W flank. On 17 March, the cone was approximately 20 m high and another explosion crater had formed on its E flank. By mid-March, the activity had begun to be visible from Guatemala City (about 40 km E) on clear days.

David Harlow climbed Pacaya during the night of 10 February and observed a slow-moving lava flow emerging from a vent between MacKenney Crater and the August 1984 lava NNE of the crater. When Enrique Molina and Randy White visited the volcano at the end of February, two vents were active in MacKenney Crater: the W vent had built a hornito and was spattering every second or two; only steam was emerging from the E vent, but it was glowing red at 6 m depth. By 17 March, a new cone over these vents had grown to 20 m height.

Enrique Molina and Michael Carr climbed Pacaya 24 April and observed continuing activity from the new cone. Small Strombolian explosions ejected tephra to 20 m height about every 7 minutes from one or both of a pair of vents 6-7 m apart. Activity was more vigorous from the W vent; only minor spattering occurred from the E vent. A steep lava tower, 3-4 m high, had grown over the E vent. Most of the ejecta were black scoria, as large as 1/3 m in diameter, that contained a few small plagioclase, olivine, and pyroxene phenocrysts. Considerable quantities of Pelé's Hair were found near the vents. The lava flow that was emerging from the E flank of the cone in February was no longer active. Farmers reported an active lava flow on the W flank of Pacaya, but the geologists did not visit the area.

Pacaya has been almost continuously active since 1965. Strombolian activity, sometimes accompanied by lava flows, began in February 1981 and increased in March 1983 [from 11:12]. By the end of 1983 a large cone had been built in MacKenney Crater. Lava emerged from various vents between there, an older cone to the north (Cerro Chino), and the nearby somma wall through early 1985.

Alfredo MacKenney reported that between 5 May and 28 July, 1985, the two cones that had formed earlier that year (one within MacKenney Crater and the other on the E wall) had increased substantially in size because of the continuous moderate explosive activity. On two occasions (30 June and 28 July) lava flows emerged toward the N and SW. On 11 August, another crater had formed between the two earlier cones, which maintained strong explosive activity until 13 October. From 19 October until 10 December, renewed activity occurred from the E cone, building a perfectly-shaped cone that reached the height of Pacaya Crater (2,500 m). Between 9 December 1985 and 16 February 1986, a large upper crater formed in this cone, which maintained strong constant pyroclastic explosions visible from Guatemala City (~ 25 km NNE). On 19 January a small lava dome had formed at the W base of MacKenney Crater, and from there lava flows extended N, S, and W. On 2 February, a large lava flow was moving W and on 2 March another was advancing S. As of 9 March, explosive activity continued to eject scoria and gases from the upper crater of the new cone, and lava flowed from its NW base.

Two cones have been growing within MacKenney Crater in October and November. On 18 and 23 November, explosions alternated at about 30-60-second intervals from the two cones, 3 and 5 m high and about 10 m apart. Ejecta from the new cones fell on the outer flanks of MacKenney Cone on 23 November. Narrow lava flows on the W flank of MacKenney Cone were observed on 16 November. The highest vent feeding the flows was about 300 m below the summit. Lava extended as much as 500 vertical meters downslope.

On 21 January at 1520 lapilli and blocks were ejected onto the N flank [see also 12:1]. In the town of Calderas (roughly 3 km N of the vent), 12 people were injured, the zinc roofs of 25 houses were perforated, and coffee crops were damaged by 1/4-kg blocks about 25 cm in diameter. Residents of Calderas quoted by UPI said that blocks obstructed roads to flank villages. Blocks also caused several grass fires and killed some animals. In Santa Elena Barillas (roughly 6 km NE of the vent), Mesillas Bajas (roughly 5 km NE), and Mesillas Altas (roughly 4 km NNE), 3 cm of ash fell. Newspapers reported that rescue workers were trying to evacuate about 100 residents from a sparsely populated area near the volcano. As of the morning of 22 January the volcano was quiet.

The explosion that ejected lapilli and blocks onto the N flank on 21 January was preceded by about 24 hours of low-level tremor, which strengthened about 15 minutes before the explosion. Tephra fell to the NE, with the 5 mm isopach about 15 km from the vent. Ash reached Chiquimula, 180 km to the ENE. Geologists visited the volcano and found a new crater about 160 m in diameter within MacKenney Cone. The small spatter cones that had been growing there in October and November had been destroyed.

A second explosion on 25 January was also preceded by about a day of tremor, initially banded, then increasingly vigorous for about 12 hours. During the 2.5 hours before the explosion, peak-to-peak amplitude reached 8 mm. The explosion, at about 1730, ejected a column that rose to 8 km altitude and was blown south by 70 km/hour winds. Many houses in El Caracol (3 km SW of the vent), El Patrocinio (3.5 km W), and Los Pocitos (6 km SSE) were hit by 25-cm blocks. The 5-cm isopach was about 4 km S of the crater. Tephra volume was probably less than 5 x 106 m3. Lava flowed S but remained far from villages. The press reported at least 15 injuries (but only one that required hospitalization) and as many as 3,000 evacuees. The next day the head of the National Emergency Committee said that 600 people remained evacuated, 63 homes were damaged, and many cattle and pigs had been killed. As of mid-February, typical Strombolian activity had resumed, and no additional strong explosions had been reported.

After about 3 hours of increasingly vigorous harmonic tremor, a strong eruption occurred on 14 June at 1930. At El Caracol, 3 km SW of the crater, 10 cm of ash fell in 3 hours, collapsing the roofs of two houses. About 600 residents were evacuated. Lava flowed SW and was continuing to advance two days later. As of 16 June, an ash column rose 2-3 km above the crater and tremor of 15 mm amplitude was continuing.

A strong 14 June explosive eruption was preceded by continuing Strombolian activity. During May, weak to moderate explosions from MacKenney Crater ejected pyroclastic material and gas in a dark column that sometimes rose 40-50 m and was carried S by the wind. Most of the pyroclastics fell within the crater. At the end of May, 80-100 explosions were occurring/hour, interspersed with some periods of quiet. Explosions continued at that rate through early June, decreasing to 40-60/hour after 4 June. Some periods of increased activity were recorded until 13 June, when a clear increase was observed.

During May, a lava flow emerged from the N flank of the cone, 80 m below the active crater. It moved down one or two narrow, well-defined channels to the saddle between MacKenney Cone and an older cone just to the N (Cerro Chino), where it spread, then continued SW over older flows to 1,800 m altitude. In some places, flow thickness reached 10 m. On steep slopes, the lava advanced at 3 m/minute. Hours before the 14 June eruption, an observer reported that an explosion occurred from the lava flow vent.

The eruption began at about 2000. A column of ash, cinders, lapilli, and small scoria fragments rose to 3,000 m above sea level (500 m above the summit) and was carried SW by the wind. An elliptical area of 4.4 km2 was covered by 6-10 cm of sand-sized tephra and lapilli. Cinder falls were reported 100 km S of Pacaya. The amount of material ejected was less than in the 21 January explosion and may have been similar to the volume of 25 January tephra.

Seismic activity associated with the eruption was recorded by several stations of the National Seismic Network. Stations PGC (1 km N of the crater) and TER (6 km SSW) registered an increase 15 hours before the eruption, and the increase was evident 5 hours before the eruption at REC station (6 km NE). At GCG (24 km N), the maximum seismic amplitude lasted 45 minutes, longer than for the 21 and 25 January explosions.

An alarm was given to the Comité Nacional de Emergencia (CONE) 2 hours before the eruption. The town of El Caracol (3 km SW) was evacuated by CONE. There were no casualties. After the 14 June eruption, activity declined rapidly. Lava emission stopped and the number of explosions registered on station PGC dropped to 20-30/day, continuing at that rate through the end of June.

During the first 20 days of July, activity was weak, limited to sporadic explosions from MacKenney Crater. No lava flows had been observed since the strong 14 June explosive eruption. Activity increased on 23 July, to 5-10 explosions/15 minutes from MacKenney Crater as recorded by seismic instruments. About 26 July, a new lava flow was observed emerging from MacKenney Crater and advancing W. Two small vents were observed within MacKenney Crater, ejecting gas, dark gray ash and pyroclastics. Most fell back into the crater, but fresh bombs were found on the older summit cone about 100 m to the south.

Alfredo MacKenney provided the following observations of activity between 14 June and 22 November 1987.

14 June: From 0800-0900 an intense eruption sent an ash column to approximately 8 km. Lava flows descended N. MacKenney Cone was partially destroyed (figure 2). 5 July: A small cone was forming on the crater floor as its four craters ejected scoria and ash. 2 August: The new cone surpassed the borders of MacKenney Crater and was visible from the summit of Pacaya. Lava began flowing SW from the base of the new cone. 30 August: The growth of the new cone was noticeable. It had four active vents. 13 September: The new cone had continued growing. 25 October: The new cone occupied almost all of MacKenney Crater. Explosive activity persisted in the cone's 3 upper craters. From the E base the cone had reached 50 m height. 22 November: The new cone had surpassed the height of the previous cones that had, in 1775, reached the altitude of Pacaya's summit crater. Lava continued to flow SW from the base of the new MacKenney cone (figure 2).

Figure 2. View from Pacaya's summit showing the effect of 14 June 1987 explosive activity on MacKenney Cone (top) and the re-formation of MacKenney Cone through 22 November 1987. Courtesy of Alfredo MacKenney.

Strombolian activity continued in January. Explosions accompanied block ejections and two small lava flows to the S. The active cone has been growing steadily since an intense eruption on 14 June 1987 that partially destroyed it.

The geologic team that visited Tacaná on 27-28 January climbed Pacaya on 31 January. From the summit of Cerro Chino at 1330, they saw frequent ejections of steam and ash, and, less frequently, bluish gas plumes emerging from MacKenney Crater.

Periodic observations of Pacaya's activity were made during February by scientists listed below. Strombolian explosions ejected blocks and incandescent bombs from MacKenney Crater every few minutes, accompanied by detonations. During the evening of 13 February, detonations were audible from Guatemala City, 25 km NNE. Although the SW flank lava flow was not visible from Pacaya's summit, observation of nighttime incandescence on 22 February (from the road below San Vicente Pacaya) indicated that it remained active.

Strombolian eruptions accompanied by lava extrusion continued at Pacaya during 1988. The SW flank lava flow observed in February remained active for about 2 months. In March and April lava extrusion began from new vents about 200-400 m below the summit. Lava effusion stopped in May, but resumed 26 May from new vents on the SW flank and continued into June, feeding flows that traveled about 1 km down the W, WSW, SW, SSW, and S flanks. Counts of explosions derived by Otoniel Matías from INSIVUMEH seismic records (figure 3) indicate that explosions increased from a few hundred/day in early June to about 2,000/day in mid-June. Ash fell 2.5 km from the volcano on 11 June. Explosions dropped to a few hundred/day in July and lava effusion stopped from all but one vent. Activity remained low in August, although a SW flank lava flow was emitted on 23 August. A lava flow formed on the NW flank at about 2330 on 6 September, and was active until 18-19 September. When the seismograph resumed operation in mid-September, daily explosion counts ranged from about 1,500 to 2,300. On 20 September a new lava flow began on the S flank at about 2,100 m altitude. Activity increased late September to early October, when some seismic events were felt on Cerro Chino (on the NW somma rim) and rumblings were heard in the S part of Guatemala City. A new lava flow was emitted 4-5 October from a vent at 2,070 m on the SW flank. The daily number of explosions peaked at 3,300 on 5 October, and gradually decreased after 10 October to fewer than 1,000 by 29 October, when lava emission ceased.

Figure 3. Estimated number of explosions/day at Pacaya, June-October 1988. Explosion events were counted on a vertical section of each day's seismic records from station PCG (on Cerro Chino, 1.5 km NW of MacKenney Crater) then extrapolated to yield 24-hour values. Upward-pointing arrows indicate the appearance of a new lava flow, downward arrows the cessation of feeding for a previously active flow; 2-headed arrows indicate the same observations but for two flows. Courtesy of Otoniel Matías.

During fieldwork 12-26 November on the S side of Pacaya, geologists observed MacKenney Crater during another period of increased activity. On 20 November the active vent was seen from the summit of the 1775 cone. More or less continuous weak Strombolian explosions ejected bombs to a few tens of meters above the crater rim. These were punctuated several times a minute to every few minutes by detonations from stronger explosions that ejected incandescent bombs and blocks to 100-200 m height from a vent at the NW end of MacKenney Crater. Some incandescent blocks rolled 400 vertical meters down the flanks of the cone as far as the moat between MacKenney Crater and the N somma rim. Incandescent ejecta was sometimes visible in daylight from the somma rim in contrast to the less vigorous February activity, and daytime explosions could also be seen from Guatemala City, 25 km NNE.

An area of active fumaroles was located on the N flank about 100 m below the summit. The accumulation of near-vent ejecta in the absence of major explosions since February had considerably narrowed the crater's width and recently raised MacKenney cone to an altitude slightly above that of the 1775 cone, which had formed the highest point on Pacaya at 2552 m. A lava flow originated in early November from a vent high on the SW flank at about the elevation of the saddle between the two summit cones. Lava flowed in several channels to about 2,000 m altitude on the SW flank, and extrusion was continuing on 26 November.

Vigorous explosive activity from Pacaya 7-10 March deposited ash to the S coast (more than 50 km away), and lava flows advanced several kilometers from two flank vents. Activity at Pacaya in recent months had been characterized by moderate summit Strombolian activity and production of small lava flows from upper flank vents. About 1600 seismic events/day were recorded through most of February. A slight increase in seismicity was observed 25-27 February, and the number of events varied from 1700 to 2300, 28 February-6 March. A logarithmic increase began 7 March at about 0600, continuing until the onset of the main eruption at about 1300 (figures 4 and 5).

Figure 4. Estimate of the number of explosions/day from Pacaya, November 1988-7 March 1989. X's mark direct observations, squares indicate data from seismic recordings. Filled squares with arrows indicate the appearance of lava flows, with the downward-pointing arrow showing the time when feeding of flows stopped. Courtesy of INSIVUMEH.

Figure 5.top Number of Pacaya's explosions counted from seismic records in a selected 15-minute interval every 2 hours, 5-7 March. bottom Visual observations of explosions during selected 15-minute intervals, 6 March until the onset of the eruption. Note that the y axis is logarithmic. Courtesy of INSIVUMEH.

On 7 March at about 1030 Pacaya was erupting normally, with small Strombolian bursts every 2 minutes from MacKenney Cone. At around 1100, fumaroles on the E face of MacKenney were emitting about 5-10 times as much vapor as usual. A large white steam plume emerged from MacKenney Cone at about 1130 and was carried NE by the wind. Strombolian explosions had stopped. Explosions from MacKenney crater at about 1230 ejected a reddish-brown ash cloud, consisting of lithic fragments produced during crater enlargement, that rose about 1,000 m. At about 1300, MacKenney Cone began to erupt dense dark ash that rose 2,000-3,000 m above the vent and was carried NE. During the afternoon, incandescent tephra rose to an estimated sustained height of 2,000 m, punctuated by explosions that ejected black ash and block-sized tephra to 3,000 m above the crater. Sheet-like flows of incandescent material and other debris, apparently fed by column collapse, moved about 200 m down the upper flanks on all sides of the summit area, exhibiting serrated fronts. Light ashfall mixed with rain occurred at Cerro Grande, El Chipilinar, San Carlos, and Santa Elena Barillas (all within 7 km and NE of the crater). At 1930, light ashfall was reported in the Barberena and Cuilapa areas (30-35 km SE). Lapilli and ash began to fall in the El Caracol area (3.5 km SW of MacKenney crater) at 1945. Bombs and blocks as much as 1.5 m in diameter fell in the La Meseta area 1-1.5 km N of the vent, forming impact craters up to 3-4 m in diameter and 1-1.5 m deep. By 2000, the ash plume was rising 2,400 m above MacKenney crater. Small pyroclastic flows were observed at that time on the SW flank, descending to about 2,000 m elevation. Explosive activity declined after 2000 and no more pyroclastic flows were observed.

Between 1230 and 1245, Zurgen Philipp observed a bulge forming on the N flank at about 2,200 m elevation, about 20 m E of the post-1983 hornito on the somma floor. As vigorous ash emission began from MacKenney Cone at about 1300, lava broke out from the bulge and flowed W (figure 6), ponding to about 10 m depth on the somma floor before overflowing toward the town of El Patrocinio (3.5 km W of the summit). At least two more vents opened at about 1,900 m elevation between Pacaya and Cerro Chino, contributing lava to the flow. Five lobes of aa, 2-3 m thick, advanced along a front about 0.4 km wide at a mean velocity of 20 m/hour. The flow burned trees and young coffee plants along its N side and cut a small road, but did not approach any houses. By 2000, the flow was ~ 1 km E of El Patrocinio and was moving W at 4-7 m/hour. Temperature measurements by thermocouple on 7 and 8 March yielded flow temperatures that varied between 860° and 1,018°C. The Comité Nacional de Emergencia and INSIVUMEH evacuated the 120 residents of El Caracol. Evacuation of El Patrocinio (population 900) was considered, but the flow's rate of advance had slowed to about 1 m/hour by 1500 on 8 March and officials decided not to require its evacuation. The Comité de Reconstrucción Nacional is relocating the people of El Caracol to a new village site.

Figure 6. Sketch map of Pacaya and nearby towns. The approximate position of lava flows is shown in black, and distribution of bomb and block impact craters is indicated by the dotted area extending NE from MacKenney crater. Contour interval, 100 m. Contours around MacKenney crater are approximate. Roads are shown by dashed lines. Courtesy of INSIVUMEH.

On 8 March at about 0815, activity increased again. Explosions from MacKenney crater fed an ash cloud that rose 1,000-3,000 m above the crater. Strong winds carried the cloud SW over El Caracol toward Escuintla (20 km from Pacaya). For 3-4 hours during the afternoon of 8 March, summit tephra production declined to moderate Strombolian explosions typical of Pacaya's long-term activity. Then lava fountaining, accompanied by increased seismicity, resumed over a period of hours in late afternoon when loud explosions were heard at a rate of about 20/minute. Activity decreased slightly early 9 March, but summit fountaining, explosions, and felt detonations continued. A column of sand- to block-sized tephra rose continuously from the crater to about 2,000 m height. Light ashfall on the S coast and near the Guatemala-El Salvador border (more than 80 km from Pacaya) was reported 9 March. Ash eruptions and strong NE winds persisted through 10 March, but activity had ended by the morning of 11 March.

A second lava flow began to emerge from a vent at 1,500 m altitude on the SW flank on 9 March at 1515. By 1930, the flow had moved about 1 km SW, and by 1640 the next day its front had reached 1260 m elevation while continuing to advance at 7 m/hour. The flow had reached 1220 m elevation by noon on 11 March, but was moving at less than 1 m/hour, and by 12 March it had stagnated. The flow cut a road and water line but did not reach inhabited areas.

During the eruption, about 75 m of the summit of MacKenney Cone was destroyed (figure 7). The crater's diameter was enlarged from 50-75 m before the eruption to an elliptical crater with horizontal dimensions of about 200 x 300-350 m. After the eruption, the crater floor was about 50 m below the rim. A single vent on the E end of the crater floor produced a strong white gas plume that rose several hundred meters.

Figure 7. Profile of Pacaya looking south from Cerro Chino on 6 and 11 March, showing the changed morphology of MacKenney Cone. Courtesy of INSIVUMEH and the Univ of Puget Sound.

Images of the eruption from NOAA's GOES satellite were obtained starting 9 March at 1800 (table 1). The plume extended roughly WSW-SW to 20 km off the Guatemala coast and was at least 3000 m high. On 10 March, surface winds blew the lower part of the plume S, while upper level winds (3000-5500 m altitude) carried the upper portions of the plume to the SW. At about 1300 the plume appeared to separate from the volcano. No plume was evident 11-13 March.

Gas bursts from MacKenney Crater's inner crater vent were observed during fieldwork between 1500 and 1730 on 4 January. The gas bursts, of varying intensity and duration, occurred about every 15 minutes. Some were accompanied by Strombolian spattering; the most energetic lasted several seconds and ejected spatter to 40 m height. A mound of agglutinated spatter was centered around the 1-m-wide vent. The inner crater was about 60 m in diameter and 30 m deep, with its S wall against the inner wall of MacKenney Crater. Rockfalls were frequent on the steep inner crater walls. Near the inner crater rim, the floor of MacKenney Crater was muddy and strewn with small blocks of spatter. Residents of the area said that this part of the crater floor had recently been occupied by a small lake. Active fumaroles were abundant along the upper E wall of MacKenney Crater. No active lava flows were evident.

Activity had been limited to degassing from fumaroles since the March 1989 eruption. Moderate steam emissions and a rumbling noise were noted during a summit climb on 29 November. Deep rumbling noises and a weak red glow within the crater were reported by nearby residents at the beginning of January, and geologists observed gas bursts and Strombolian spattering during 4 January fieldwork. Observation trips on 27 January and 4 February revealed a growing cone on the floor of MacKenney Crater, with intermittent explosive activity that occurred every 2-5 minutes.

Volcanologists from INSIVUMEH and Michigan Tech visited Pacaya on 13, 14, 17, 18, and 28 February and 1, 2, 3, and 4 March, and flew over the volcano on 16 February. The following is from their report.

"Activity at Pacaya continued at a low level, consisting of brief (10-60 second), weak (ejecta typically thrown 2-100 m), Strombolian explosions with reposes of <1 to several minutes. All activity was from a small cone, 6 m high and 8 m wide at its rim, within MacKenney crater. The explosions were accompanied by gas emission (with jet-like noise) and often by fine ash clouds.

"On 17 February, during activity that was typical of the observation period, 78 COSPEC scans were made from a ground observation site 1.25 km from MacKenney crater (at Cerro Chino). Pacaya was emitting SO2 at an average rate of 30 t/d, with the measured range varying between 3 and 130 t/d. Higher fluxes were directly associated with observed small explosions. The new SO2 observations at Pacaya were much lower than values measured several times from 1972 until 1980 (Stoiber et al., 1983; reference under Santiaguito), which were generally between 250 and 1,500 t/d."

Strong Strombolian activity that fed a tephra column 4.5 km high began at 1130 on 14 July and continued for about 3 hours. Lapilli fell S of the volcano and onto the towns of El Patrocinio (3.5 km W of the crater) and El Caracol (3 km SW). Two small lava flows were extruded, extending about 150 m to the N (roughly 200 m wide) and SW (about 10 m wide). The activity destroyed the small cone that had been growing within MacKenney Crater and ejected debris that had clogged much of its floor, leaving a significantly larger crater.

Press sources reported that an eruption of lava, gases, and ash occurred during the night of 16 September. The activity was attributed to the opening of a new crater. A "state of alert" was declared by INSIVUMEH due to the activity; no damage or injuries were reported.

Fieldwork was conducted at the volcano by a group from the Univ de Genève (mid October-4 November), and scientists from Michigan Technological Univ and Guatemala (25 November).

The following is modified from a Univ de Genève report. Strombolian activity increased during October, to >400 recorded explosions/day. Scientists visting the volcano observed explosions every 30 seconds to 5 minutes (17 and 21 October), and counted up to 17 explosions during a 15-minute period (28 October). The explosions ejected incandescent material to 10-50 m above the 25-m-tall cone in MacKenney crater, and were visible on clear nights from Guatemala City (25 km NNE). The number of seismic events (recorded by a joint INSIVUMEH-Univ de Genève digital seismic station) increased steadily, from 147 on 13 October to 457 on 20 October, and averaged over 450 daily during the following several days. Tremor was recorded during periods of closely spaced explosions (including the 28th), with 5 hours of continuous tremor recorded on the 20th. As of 4 November, two lava flows were moving down the N flank of the volcano, and explosive activity was unchanged.

The following report is by Michael Conway. "Vertical Strombolian eruptions (lasting from seconds to <1 minute) from the crater of MacKenney cone occurred every 3-5 minutes, hurled incandescent bombs (to 2 m in diameter) to 150 m above the vent, and were accompanied by a jet-like sound. Black eruption clouds, with the coxcomb geometry characteristic of phreatomagmatic blasts, were rarely observed, suggesting that the conduit is moderately well sealed from infiltrating meteoric waters. Low-temperature fumaroles (up to 100°C) were active on the E summit of MacKenney cone, and patches (areas of one to tens of m2) of yellow sublimates were common.

"An aa lava flow was being extruded from a pit crater (50 m long, 10-15 m wide, and 2-5 m deep) on the N flank of MacKenney cone, 40-65 m below the summit; opening of the pit crater may be related to an eruptive episode that occurred on 16 September. A lava channel, 5 m wide and 2-4 m deep, delivered lava to the low-lying area between MacKenney cone and Cerro Chino. The lava flux was variable and flow velocities ranged, roughly, from 1 to 6 km/hour. Collapse of the lava flow front was common (slopes of the cone are about 30°), exposing fresh lava and sending hot block avalanches down the channel. The latest stage of lava flow activity began on 3 November and has erupted two flow units, one of which was still active. Lava flows erupted since March have effectively armored the N-central flank of the cone. A result of continued construction of MacKenney cone is that, for the first time, the cone is visible from San Francisco de Salas (2.5 km downslope, to the N)."

Vigorous Strombolian activity continued during December and early January. Explosions occurred at a rate of about 1-2/minute in one 38-minute period of detailed observation during a 5 January summit visit. The explosions hurled incandescent globs of magma to 15-100 m above the crater. Four different lava flows were observed moving down the N slope of MacKenney Crater, before turning W at the break in slope with older lavas. The oldest of the moving flows was aa to blocky lava, which had apparently been active for several days before the visit. This flow was 450 m long, 35 m wide, and 4 m thick, with well-developed levees, and had essentially been separated from its source. Its front was at the base of the steep slopes of MacKenney Crater, where thick and extensive deposits of post-1987 lava created a nearly level flow field. At that point, it was actively collapsing and moved at about 1 m/hour during 5 hours of observation. One incandescent block that spalled off had a temperature of 967°C and a somewhat plastic character when struck by a heavy hammer.

A fast-moving, new flow was observed traveling 15 m/minute in the same channel as the oldest flow, but had not yet advanced to more than half the length of that flow. Lava flows and Strombolian explosions were again visible from Guatemala City in the early morning of 14 January.

The inactive crater and the small crusted-over lava lake, on the N side of MacKenney Crater, were also visited on 5 January. Two smaller lava flows were being emitted from the lava lake, just E of the second, fast-moving flow. These flows had reached lengths of 150 and 300 m, and one was readily accessible. Its temperature was 1086°C and it flowed from its vent at a rate of 6 m/minute. A 2-m-tall, sublimate-encrusted hornito, located near the lava lake, emitted 957°C gases rich in HCl.

Strombolian eruptions occurred every 1-3 minutes and small lava flows were active during 3 hours of fieldwork on the morning of 6 February and a 10 February overflight by INSIVUMEH and Michigan Technological Univ volcanologists. Similar conditions had been reported on 5 January (15:12). The lava flows emerged from a S flank vent about 100 m below the summit of MacKenney Cone. Substantial growth during the past year had built MacKenney Cone to several tens of meters above the pre-1965 summit crater.

SO2 emissions (determined by COSPEC from neighboring Cerro Chino cone) averaged 23 ± 11 t/d on 6 February, with a range of 4-60 t/d. The previous measurements, on 17 February 1990, yielded a mean of 30 ± 28 t/d and a range of 3-131 t/d (15:03).

In comparison with observations made in early February (16:02), visits to the volcano in mid-March-early April revealed a decrease in eruptive activity. A small vent with night glow on the W flank (50 m below the summit), periodically the source of incandescent lava fragments that rolled down the upper flank, had disappeared by 21 March. Strombolian activity from a cinder cone in the W quarter of MacKenney Cone's 1987 crater ejected material to 100-150 m height. The number of explosions declined from about 20 to 1-2/hour over the mid March-early April observation period, and during the first week of April, the primary ejecta changed from lava spatter to ash. Some collapse occurred on the cone's interior walls. Two explosions, observed during a 3-hour period on 10 April, emitted ash clouds hundreds of meters high. Lava flow activity, prominent from mid-November through February (15:11-12 and 16:02), declined, and ceased entirely by 10 April. A decrease in seismicity, coincident with the decrease of eruptive activity, began about 1 April and continued as of 19 April.

Fourteen eruptions occurred during the most recent phase of strong explosive activity, 6 June-1 August, with the strongest and most destructive activity occurring 27-31 July. Activity was at low levels as of 15 August.

The following report from Philippe Rocher describes activity through mid-June.

"During the first half of 1991, activity was continuous and relatively quiet, with several small eruptions and lava flows from the main crater. This last cycle of activity began in November 1990. The continuous ejection of material built a cone that reached 400-500 m height. Although seismicity showed no significant changes in May, occassional pulses of increased surface activity occurred. On 11-15 May, explosion counts ranged from 1,170 to 1,730/day and a new lava flow was emitted. The cone reached 500 m high and lava traveled down the SE slope.

"On 6 June, explosive activity increased again, with explosions every 10-40 seconds and ash reaching 100-500 m heights. The next pulse occurred on 11 June. On the following day, strong explosions sent material to 500 m height and triggered avalanches that destroyed the summit of the cone. Lava flowed down the SW slope. Ash emissions to 500 m height and short lava flows characterized the next increase, lasting 4.5 hours on 14 June. On 16 June, a 10-hour episode of strong explosions ejected a black plume to 600 m height and caused avalanches that traveled to the foot of the volcano. Between the different eruptions, strong degassing continued, accompanied by B-type earthquakes and small, low-amplitude (about 1 mm) tremor episodes."

The following is from Eddy Sánchez.

"The most explosive and destructive activity during the current phase of activity began at 0100 on 27 July. Strombolian activity destroyed the main crater, and ejected ash and lapilli to the SW, principally affecting Caracol, Rodeo, and Patrocinio, the same towns affected by the eruption on 25 January 1987. Activity decreased at 0230." The press reported that three people were injured and 2,000 left homeless.

"Intense activity resumed at 1330-2230 on 30 July, with four cycles of moderate explosions, each cycle lasting 1.5 hours. Similar activity occurred the next day, when columns of fine ash and gas rose 400-1,000 m above MacKenney Crater. The last strong episode of Strombolian activity began at 0230 on 1 August, when ash clouds reached 700-1,000 m heights, with pulses and pauses of 30-60 minutes, and blocks (>=5 m in diameter) were ejected onto the flanks of the volcano.

"Local agriculture was significantly damaged by airfall from this recent phase of explosive activity. Corn and bean fields were destroyed, as well as part of the coffee crop. Airfall thicknesses ranged from 0.5 to 26 cm, with up to 5 cm in Rodeo and 15 cm in Santa Lucía Cotzumalguapa (figure 8). The ash was deposited as far as 55 km WSW (Pueblo Nuevo Tiquisate).

"During the last eruption, on 1 August, INSIVUMEH recommended to emergency agencies that the approximately 1,500 residents of Caracol, Rodeo, and Patrocinio be evacuated, due to the hazard of a new violent eruption. The next day, seismic and eruptive activity decreased considerably, allowing the evacuated people to return home. Activity continued to decrease quickly, with 40 B-type microearthquakes (frequency, 4-5 Hz, and amplitude, 2.0-2.5 mm) recorded daily on 7 August. Activity as of 15 August was considered at low levels."

A strong explosive phase occurred on 22 August, after 3 weeks of relative quiet that followed strong explosive activity from 6 June to 1 August. Volcanologists visiting the summit observed an increase in fumarolic activity beginning at 1400, and the start of small explosions 20 minutes later. The frequency and sound of the explosions increased, and by 1425, lapilli and blocks were rising above the rim of the crater, before falling back into the interior. Between 1,500 and 1630 the explosions increased in size, feeding a continuous lava fountain 100-200 m high. The eruption entered its most vigorous period at 1700, with a black plume 1,000 m high, and explosions heard to 1 km from the crater. Activity declined 1710-1730, as lava fountaining was replaced by strong black plume emission that gradually ceased.

The summit was visited again on 20 September, two days after strong explosions were observed from Guatemala City (45 km N). Fresh bombs, flattened on impact, with crystal-rich interiors, covered the E part of MacKenney Cone. The explosions on 18 September coincided with the last in a 24-hour series of earthquakes that killed at least 25 people. The largest shock [M 6.1 at 5 km depth] was centered about [30 km NNW] of the volcano.

The following, from Michael Conway, describes 11 November fieldwork by geologists from INSIVUMEH and MTU. On 10 November, a small-volume lava flow began to erupt from the summit of MacKenney cone at about 1600. By the evening of the following day, the flow had traveled 50-100 m down the cone's S flank. This was the first time in several years that a flow was erupted onto the S flank, indicating that the powerful explosive eruptions of July and August resulted in major changes in the vent geometry of MacKenney cone.

Several small eruptions observed on the afternoon of 11 November produced black plumes that rose as much as several hundred meters above the vent, generally accompanied by a jetting sound. Widely spaced (50-125 m) en echelon fractures were identified on the W flank of MacKenney cone. The fractures began 10-30 m below the summit and continued down the flank for roughly 50-100 m. A series of point-source fumaroles marked the fractures, which may have originated as the W part of the cone sagged into an evacuated shallow magma chamber following the powerful pyroclastic eruptions of July-August.

During 1991, 14 eruptive episodes were documented at Pacaya, with the strongest in July and August (figure 9 and 16:7 & 9). Continuous gas emission, punctuated by occasional explosive activity, characterized the activity until 28 September, when explosions began to eject pyroclastic material to 25-35 m above the main crater. Weak to moderate explosions were more frequent in October. Extrusion of lava onto the SW flank began on 27 October, and this flow remained active as of early January.

Strong explosions on 8 January ejected substantial amounts of pyroclastic material to 200-400 m height. The explosions destroyed part of the active crater, and were accompanied by acoustic waves that were heard and felt over a radius of 15 km. Seismic activity increased for 7 hours on 8 January, with 80-150 recorded explosions/hour accompanied by tremor of constant frequency and higher amplitude. Explosion shocks and tremor declined after the 8 January activity. INSIVUMEH's volcanology section notified the Emergency Committee and recommended that appropriate precautions be taken. Preparations were made to evacuate the 5,000 residents nearest the volcano, but activity declined and none were evacuated. Vigorous explosions resumed on 13 January.

Activity was unusually high through May, with several thousand explosions recorded seismically every day (figure 10). Powerful pyroclastic episodes in early May temporarily forced the evacuations of villages near the W base of the volcano. During the first week of May, two lava flows were extruded from vents near the NW and S summit of MacKenney cone.

Pacaya has erupted almost continuously since January-February 1990, when Strombolian activity was observed producing a new cone. Strong Strombolian activity destroyed the new cone and lava emission began in July 1990. Since then, lava emission has continued, and periodic increases in explosive activity have resulted in crop damage and the evacuation of up to 1,500 people.

A new cycle of activity has begun since June, with increased energy released by explosions. Continuous degassing from the active (MacKenney) crater produced sustained whitish columns that rose 200-300 m. In July, explosion seismicity increased, with explosions occurring at intervals of 5-30 seconds (figure 11). The strongest were quite energetic, ejecting abundant pyroclastic material that rose 300-400 m above the crater, and generating acoustic waves that were heard and felt in nearby towns. A moderate amount of ash was carried about 5 km W and NW by winds (toward El Patrocinio and El Cedro).

Activity continued through August. Increased lava extrusion 22-23 August fed a flow that advanced 750 m down the W flank. Two others traveled 75 and 250 m to the SW. Explosions, generally moderate to strong, continued at a rate of 2-5/minute, generating sound waves. The Comité Nacional de Emergencia (CONE) was alerted to the possibility of a stronger eruption.

Activity intensified on 6 September between about 0900 and 1700. Vigorous lava extrusion produced two flows that descended 300 and 1,150 m down the W and SW flanks, respectively, accompanied by moderate Strombolian activity. Explosions from MacKenney Crater ejected abundant ash and fine sand within clouds of white fume that reached about 500 m height. Ash fell on towns about 3.5 km W to SW of the volcano (El Patrocinio, El Rodeo, and El Caracol). Activity then returned to normal levels, about 200-300 explosions per day.

INSIVUMEH officials warned that this pattern of activity could last several weeks and could precede a much stronger eruption. The authorities and people living around the volcano were advised to remain alert to emergency pronouncements issued by CONE. Powerful pyroclastic episodes had forced temporary evacuations of villages near the foot of the volcano in early May (17:05).

During an overflight on 8 November 1993, Stephen O'Meara saw brief glimpses of MacKenney crater (figure 12) through breaks in the clouds. A small cone was seen at the center of the horseshoe-shaped MacKenney crater, which was breached to the S. Incandescent lava issued from a vent near the SW wall of MacKenney crater and flowed SE across the summit crater floor. The SE slope was covered by a fresh lava flow with overlapping lobes that extended two-thirds of the way down the flank. On 10 November, ash eruptions originating from one or more vents in MacKenney crater were observed from the S near Los Pocitos at intervals of about 30 seconds, with larger explosions occurring several times a minute to once every five minutes. At night three incandescent lava channels could be seen descending the SE flank.

Participants of the Santa María Decade Volcano Workshop observed MacKenney crater ash eruptions from vantage points on Pacaya's N caldera rim. The previous night, 12 November, strong detonations from Pacaya were heard in Antigua, Guatemala, 25 km NW. Following destruction of the upper cone by strong explosions in 1989 and 1991, MacKenney cone had rebuilt itself to a height comparable to that in 1991. Its summit was slightly higher than the pre-1965 cone immediately to the E (figure 12), and was again visible from San Francisco de Sales on the N flank. A line of fumaroles marked the mostly buried N rim of the 1989 crater. Fresh lava flows with pronounced levees from eruptions in 1990 and 1991 coated the N and NW flanks, and covered the moat floor between the caldera rim and MacKenney cone, almost burying the upslope side of the prominent spatter cone from a 1984 eruption (figure 12).

Explosions several times a minute to every few minutes ejected blocks and ash to heights of about 150 m, accompanied by detonations. Larger pulsating ash columns were also seen punching through the persistent summit cloud cap during brief observations the following day from 20 km E on the road to Cuilapa.

Summary of 1992 and 1993 activity. Following a period of increased activity June to September 1992 (17:08) explosive activity declined, although lava flows continued to travel about 1.5 km to the SW.

An increase in the amplitude of explosion seismicity took place at the end of October and beginning of November 1992. After 15 November, the frequency of explosions increased (figure 13), reaching 4,800-5,000 explosions/day around 25 December, and 7,800 explosions/day in early January 1993.

On 10 January 1993, a large explosive eruption lasting 4 hours was accompanied by collapse of the S rim of MacKenney crater, producing a hot avalanche that traveled 3.5 km to the SW, cutting the road between El Caracol and Los Pocitos. The crater-rim collapse, the first since the birth of MacKenney crater in 1961, involved an area 75-80 m wide and 30 m deep. During the eruption, a small lava flow traveled down a 7-15 m deep channel formed by the crater-rim collapse to reach 1,600 m elevation.

Column collapse produced a nuée ardente that traveled 2 km SE to 1,600 m elevation, leaving a deposit just 7-10 cm thick. The color of ash in the 4-km-high eruption column changed from gray to red as it passed through a rain cloud. Ashfall was distributed NW to a depth of 6 cm at a distance of 5 km from the crater. Traces of ash fell in Guatemala City and beyond, causing some damage to jet aircraft turbines. Ashfall burned leaves to 2.3 km NE of the crater and dehydrated leaves to 5-7 km away, which later fell. Residents of villages in this region suffered irritation to the eyes and nose. In the two weeks following the eruption, wind-blown ash from the nuée ardente deposit reached heights of >3.5 km and was redistributed 40-45 km to the SW.

INSIVUMEH and CONE (Comitte Nacional de Emergencia) had predicted a strong eruption 3 days prior to 10 January, and the villages of El Caracol (3 km SW) and El Patrocinio (4 km W) were evacuated during the eruption. Following the eruption, typical activity continued at Pacaya, with the daily number of explosions decreasing intermittently from 7,000 to about 3,000/day by mid-year (figure 14). An increase in the amplitude of explosion seismicity in mid-May was not accompanied by stronger eruptions. Lava flows traveled almost continuously for about 1 km down the S flank from a source just below the breached S crater rim.

On the evening of 21 September, a vertical eruption column 2-2.5 km high with a 300-400 m incandescent base was accompanied by vigorous lava effusion and strong tremor. Lava fountains reached 700 m heights at the beginning of the 7-hour eruption, and were most vigorous for 9-15 minutes shortly after midnight. Ash fell at El Caracol, although no damage was recorded. Lava flows traveled 1 km to the S over the 10 January flows.

At the beginning of the second week of October the frequency and magnitude of activity increased for 9 hours. A dark ash column rose to a sustained height of 400 m, depositing ash to the W. The lower 200 m of the column, which reached heights of 700-800 m, was incandescent.

Pacaya continued to erupt on a regular basis. Between late afternoon on 5 February and sunrise on 6 February, Stephen O'Meara photographed and videotaped various stages of moderate eruptive activity from a vantage point about 4 km SSW of the summit (about half-way between the villages of Los Pocitos and El Caracol). Strombolian activity had increased markedly since he visited the volcano in November 1993. Strombolian fountains rose 100-200 m, and some of the larger ash clouds rose upwards of 400 m. An incandescent lava flow emerged from a single opening in MacKenney crater and traveled down the SW flanks. That main flow then divided into two separate lobes, each of which had several rivulets. Incandescent boulders continually rolled down the flank from the fronts of these flows. The longest flow extended to about 1,800-m elevation.

Activity was strong in the late afternoon, weak in the morning, and throughout the night eruptions occurred about once every minute. In the afternoon, the ash plumes appeared dark gray or black and sometimes there were 3-4 ash ejections in rapid succession. Stronger ejections were usually inaudible, but when visible activity waned in the early evening, noises like "rolling thunder" and "a lions roar" prevailed. At other times the observers noted noises like "the ebb and flow of waves pounding a cliff." More powerful blasts in this interval produced detectible earth movements.

Activity increased at 0400 on 12 October with vigorous Strombolian explosions. Approximately 5 cm of ash was deposited in El Patrocinio, ~4 km W (figure 12). Ash drifted as far as Santa Lucia Cotzumalguapa, ~45 km WSW on the Pacific lowlands. Although apparently declining on 14 October, Strombolian activity was continuing, an ash plume to 300 m above the vent persisted, and tremor was still being detected by the seismometer at Pacaya. As of 14 October, five lava flows active on MacKenney cone had reached the base of the edifice, two on the N, two on the W, and one on the S flank. Flow velocities were reported to be 10 m/hour. Heavy rains and cloud cover since the start of the increased activity have prevented detailed observations. The Comite Nacional de Emergencias (CONE) evacuated 142 people from the towns of El Patrocinio, El Caracol (3 km SW), and other nearby areas, to San Vincente de Pacaya (5 km NW).

Pacaya is a complex volcano constructed on the S rim of the 14 x 16 km Pleistocene Amatitlan Caldera. In 1565, the first recorded historical eruption from Pacaya caused ashfall for three days in Guatemala City. Following explosions in July and October 1965, Strombolian activity was generally continuous until March 1989 when explosive activity removed ~75 m of the MacKenney cone summit and enlarged the crater. Strombolian activity began again in January 1990 and has continued intermittently since then. This latest episode of activity, although smaller in terms of area impacted by tephra, is similar to the activity during July-August 1991, which again destroyed part of the cone and damaged towns W of the volcano.

Pacaya erupted between 0630 and 0700 on 1 June, sending up a 6-km-high plume. Hot ash burned vegetation and damaged radio-antenna equipment near the summit. A thin layer of ash extended a few kilometers from the vent, with ashfall reported 2.5 km N in San Francisco de Sales and 3.5 km W in El Patrocinio (figure 15). A plume visible at 0715 on GOES-8 satellite images was moving SE at ~19 km/hr, prompting a NOAA Volcano Hazard Alert at 1000. Analysis based on comparison of wind shear data and observed translation of the plume suggested that the plume rose to ~9 km altitude. Eddie Sanchez noted that venting ash destroyed the 1984 spatter cone (SEAN 10:03), called "El Hornito," located roughly midway between the N caldera rim (Cerro Chino) and the previously active MacKenney cone (figure 16). In place of El Hornito was a small crater venting lava, a substantial amount of which flowed S.

Otoniel Matias reported that on 10 June a small newly constructed spatter cone had collapsed. A news report described two strong explosions in the early afternoon and a thick column of smoke. Matias said that on 14 June more lava erupted, but compared to earlier phases there was less airborne ash and venting gases, and the plume reached only 150-200 m above the crater floor. In accord with this observation, SAB failed to image a plume at 0845 on 14 June; they estimate that a plume must have a height of >3,000 m for them to detect it. Matias went on to say that the 14 June eruption continued vigorously for two hours without pause. Lava continued to escape as of 16 June, and one or more lava flows had moved S for a distance of ~600 m.

Pacaya sits 30 km S of the center of Guatemala City (figure 15), with its population of >1.5 million. An eruption in 1989 produced a 4.5-km-tall eruption column that enlarged the MacKenney crater. Lava flows followed in 1990-91. Strong eruptive activity in June-August 1991 destroyed part of the MacKenney cone and damaged villages to the W. The volcano's olivine basaltic lavas have often flowed out of the collapsed SSW sector, traveling away from most of the nearby inhabited areas. The international airport (La Aurora) lies ~23 km from the crater and airplanes commonly fly over the volcano (figure 17).

Figure 17. Aerial photograph of Pacaya viewed from the NW, 10 November 1994. Vapor rising from the MacKenney summit crater can be seen flowing down the S flank. Courtesy of Stephen O'Meara.

Pacaya erupted more forcefully than usual beginning late on 10 October. Based on an INSIVUMEH report, between about 2300 on 10 October and 0200 on 11 October Pacaya produced a moderate Strombolian eruption with sustained fountaining of incandescent materials up to 500 m high.

The plume's maximum height reached ~3.7 km altitude; within that plume the ash column rose to ~700 m. During the eruption winds blew from the NNE at 35 km/hour with gusts to 45 km/hour; they carried fine ash toward the town of Esquintla. A report from Puerto San Jose, a city on the Pacific coast ~60 km SW, indicated that the earlier dark ash cloud had thinned during the day.

The explosive eruption was followed by significant lava effusion from the crater. The longest lava flow travelled SW for 1.5 km over the surface of an older flow field. At 0300 the flow front's velocity was 100 m/hour; it came within 300 m of the relatively flat area reached by the 1991 lava flow. Lava ceased venting at dawn; however, the SW flow remained incandescent and slowly moving. Although eruptive strength diminished, some tremor persisted on 11 October. On that day satellite images (Band 2 on GOES-8) showed a small hot spot. An INSIVUMEH report on 14 October noted that ongoing eruptions continued into the morning of the 12th. After that the eruptive vigor and amount of tremor both dropped and no new lava vented from the crater.

On 16 October INSIVUMEH reported that Pacaya continued to expel abundant white steam. At that time there were no audible explosions, underground booming noises, or newly vented lava flows. Tremor was present, presumably related to the degassing seen at the surface. Eddy Sanchez noted that 38 people were evacuated from neighboring villages during the height of the eruption.

INSIVUMEH reports further described volcanic activity in October (BGVN 21:09) and a more forceful eruption that began on 11 November. Seismic data were registered at a local, one-component station (exact location undisclosed).

Activity during October 1996. At about noon on 4 October the volcano produced a 20-minute- long eruption of pyroclastics that reached 100-150 m above the crater rim. Less sustained eruptions followed later that day.

A small phreatic eruption occurred the next day (between 1000 and 1330 on 5 October), throwing pyroclastics 300-400 m high and generating an ash-bearing column rising 600 m above MacKenney crater. After that, activity generally continued as smaller explosions. Specifically, these explosions occurred at intervals of 2-5 seconds, typically throwing incandescent material to 20-30 m; but at intervals of 1-3 minutes, more energetic explosions sent material to 150 m. In response to unbroken eruptions on the morning of 6 October, a sustained column developed that was 100 m tall.

On 7 October outbursts increased in vigor with incandescent discharges 70-150 m high and coffee-colored clouds blowing E. At the seismic station, strong harmonic tremor was recorded. As a result of these factors, an alert was given to authorities. On 8 October the tremor fluctuated in amplitude but continued and a sustained 15-minute eruption led to a column reaching 200 m above MacKenny crater. The vigor of eruptions dropped noticeably later on 8-9 October.

Between 2300 and 0200 on 10-11 October, a moderate Strombolian eruption took place (BGVN 21:09). Although previously unreported, pyroclastic flows during the eruption ran down Pacaya's N, E, and S flanks reaching the cone's base. Some forceful phases of the eruption sent ash to 700- and 800-m heights. Winds of 35 km/hour from the NNE, gusting to 45 km/hour, blew ash toward the towns of Esquintla, 25 km SW, and Puerto San Jose, 60 km SE. Fine ash was also detected as far away as the Salvadorian border, 120 km SSE. Tephra with grain sizes of 0.5-3 cm reached distances of 4-7 km from the source. Evacuees from the village of El Patrocinio (3.6 km W of the crater) returned on the afternoon of 11 October. That day seven hours of post-eruption tremor registered and a bluish white plume stood 30-70 m above the crater rim. The volcano was quiet between 12 and 13 October except for 30 microseisms with 3-7 second durations.

A swarm of microseisms during 0900-1900 on 14 October consisted of 70 events of 5-10 second duration. On 15-16 October the volcano was seismically quiet but on 17 October there was an 11-hour period when seven lava avalanches broke loose. These typically took 1-3 minutes to reach the base of the cone. Five more avalanches followed on 18 October, and others followed later as freshly erupted lava and debris moved downslope. On 19 and 20 October only sporadic microseisms registered (2-15 mm peak-to-peak with 5-15 second periods); these originated at 3-5 km depths beneath the cone. Various types of weak tremor reappeared again during 21-25 October with 3- to 15-minute durations.

During 19-25 October, except for the above-noted seismic events, there were no earthquakes registered; plumes were bluish to white, and rose to 300-350 m in height. Despite technical problems with the seismograph on 26 and 27 October, some microseisms were registered; on 28 October weak tremor occurred during the entire day and 5-15 type-B earthquakes took place each hour. Elevated but fluctuating seismicity continued through the end of October.

Vigorous blue-to-white columns poured out of the MacKenny crater on 26-28 October; these were carried by 25-35 km/hour N winds. Weak fumarolic emissions took place along the N segment of the cone's base. A steamy column rose to 100-150 m high and was blown S.

Activity during November 1996. After 2 November there were repeated explosions that sent puffs of steam 30-100 m above the MacKenny crater floor. Poor visibility kept observers from seeing whether pyroclastic material was ejected; however, during 1-7 November wide steam columns reached 300 m.

The week of 1-8 November included extensive intervals of weak tremor and other minor events; in the most extreme example, on 1 November, tremor was continuous. Other events included a series of type-A earthquakes that took place in an 18-hour interval on 3 November. This swarm consisted of 63 events at depths of 200-700 m. Type-B earthquakes were also common.

On 9 November tremor was relatively scarce, only about one hour in total duration, but on 10 November four hours of tremor took place. RSAM estimates made at 10-minute intervals were constant through 0900 on 11 November when the register reset.

Initial Strombolian explosive activity from MacKenny crater began at 0930 on 11 November. Although the crater was not visible, strong-to-moderate explosions could be heard 4 and 3 km away (at the villages of El Patrocinio and San Francisco de Sales, figure 18); these continued until 1142 when a 15-minute interval of stronger explosions began that included incandescent material thrown 70 m above the crater rim; at 1157 the explosions became more frequent (4-12 each minute) and stronger (throwing incandescent material ~100 m above the crater).

During the course of the 11 November eruption lava progressively rose to the crater rim, and about midday, coincident with a sustained burst of incandescent ejections, lava spilled out over the WSW flank. The associated incandescent material rose ~100 m and a steam column rose to ~200 m. By 1330 the lava flow had reached 450 m in length traveling in a SW direction, and the column of incandescent material rose ~300 m.

At 1645 on 11 November the eruptive column rose dramatically, reaching at least 1.1 km tall. Falling ash was reported as far away as the town of Esquintla (figure 18). Meanwhile, the escaping lava flow reached 1 km in length and small tongues of lava 200-300 m in length fell from the breach in the crater. Incandescent avalanches moved down the cone's S flank and some reached as far as the base. A Red alert was recommended at midday on 11 November; evacuations followed at nearby villages. About 1900 on 11 November the lava flow reached the cone's SE base, a distance of ~2 km. About this time, large lapilli rained down on the fields and forests of the region damaging vegetation.

Figure 18 shows the isopach map resulting from these 11 November eruptions. Within the 35 mm isopach tephra ranged from 2 x 3 cm to 4 x 6 cm. Inside the <1 mm isopach there were only traces of ash. Small eruptions continued into December.

Pacaya lies 30 km S of Guatemala City, an urban center with a population of over 1.5 million people. The volcano's olivine-basaltic lavas have often flowed out of the collapsed SSW sector travelling away from most inhabited areas.

Reports by Otoniel Matías of INSIVUMEH described volcanic activity after the 11 November eruption (BGVN 21:11) until 12 December. Seismic data were registered at a local one-component station (exact location undisclosed).

Activity during 11-30 November 1996. Heightened explosive Strombolian activity from MacKenney crater on 11 November was accompanied by abundant lava flows that covered most of the SW flank and traveled as far as 2 km from the volcano. The ash column reached 2 km, and deposited a 7-cm-thick layer of coarse tephra (1-3 cm in size) over the village of El Caracol (~4 km SW of the summit). Ash transported SW by strong winds fell in Escuintla and, as some local newspapers reported, in the Mexican city of Tapachula (300 km NW). At 2010 of the same day the eruption started dying down.

For the remainder of the month there was continuous emission of white-blue "smoke" at variable heights (50-300 m) above the crater. After the 11 November eruption seismicity was dominated by type-A volcano-tectonic swarms. During the next few days the swarm's foci deepened from 0.2-1 km to 5-10 km and then to 15-20 km and finally on 18 November, to 25-35 km. On 12 November a lava tongue ~2.5 km long was flowing S toward Cerro Buena Vista. On 13 November another small lava flow ~450 m long was observed on the SW flank.

During 15-18 November there were explosions of variable intensity every 1-3 minutes; the strongest explosions lasted 25-50 seconds and had amplitudes of 35-40 mm (peak-to-peak) and frequencies of 3-5 Hz. These explosions expelled pyroclastic material 75-150 m above the crater, depositing most material inside the crater.

On 20 November the seismic activity became characterized by harmonic tremor and type-B earthquakes, indicating magma movement toward the crater. The same day an eyewitness reported a red glow inside the main crater. Early on 21 November the white-blue column rose >500 m above MacKenney crater, but later that day decreased to ~300 m.

During 26-28 November the seismicity became characterized by alternating periods of activity followed by quiet for about 12 hours. The seismic activity shifted between seismo-tectonic swarms (one event every 2-10 minutes) and continuous tremor, suggesting that magma was ascending from depth and the confining rock was adjusting to the new stress field. After 28 November the activity was again dominated by seismo-tectonic events. Originating at depths of 0.2-1 km and up to M 1, there were 467 such events during a 24-hour observation period.

Activity during 1-12 December 1996. The white-blue column observed during November was also reported throughout this period at heights of 70-250 m. On 2, 3, and 4 December small explosions every 1-8 minutes were accompanied by ejection of incandescent material up to 100 m above MacKenney crater. This Strombolian activity was associated with harmonic tremor, although such tremors had appeared since the night of 1 December.

On 5 December puffs of steam lasting 5-7 minutes built up a column that rose 400 m. On 6 December, steam-and-ash expulsions of variable intensity turned brown in color and were observed at intervals of 2-10 minutes.

During 7-10 December the seismicity was characterized by tremor (mean amplitude, 2-5 mm; frequency, 3-4 Hz; and duration, 2-15 minutes) and type-A seismic events at 0.3-1 km depth. On 10-11 December steaming was observed on the N, W, and SW flanks of the MacKenney cone.

Between 5 and 13 February graduate students from Northern Illinois University in collaboration with INSIVUMEH scientists conducted studies at Pacaya volcano. Focused fumarolic activity was observed at the summit, whereas diffuse gas emissions occurred around the SW flanks. Numerous micro-seismic earthquakes were recorded daily during this period.

Lava samples were collected from the 11 November 1996 flow. Analysis showed that the lava was a highly- vesicular, plagioclase-phyric basalt that resembles basaltic flows from previous eruptions.

While visiting Guatemala City, Kent Johnson observed small explosions at Pacaya's summit on three hazy nights between 30 July and 4 August from vantage points ~40 km N of the volcano. The explosions sent lava clots into the air and lava flows down the side of the volcano; orange streaks of lava were observed traveling a short distance down the volcano's W slope. Through binoculars, the summit appeared orange with bright orange- yellow bursts occurring at ~5 minute intervals. During the day on 4 August, puffs of steam or ash were emitted from the summit. Hazy conditions made it difficult to photograph the emissions.

According to an 11 August aviation notice, Pacaya erupted around2100 on 9 August. Surface observations in Guatemala City indicated that the volcano was in eruption although no ash plumes were discerned in GOES-8 imagery during 11 August due to thunderstorms. Another aviation notice from Guatemala City reported an eruption at 0800 on 14 November. Although an ash cloud was reported by observers, the ash was not visible in GOES-8 visible or infrared imagery due to cloud cover.

Pacaya erupted unusually vigorously on 20 May. As a result, ash falling in the adjacent highlands damaged crops; ash falling to the N choked the capital and its international airport; and ash located at about 10 km altitude entered the Gulf of Mexico and joined smoke from widespread forest fires. An airplane on final approach to landing at the Guatemala City airport hit ejecta, sustaining damage but landing without incident.

The outburst's greatest intensity occurred during the hours 1205-1900. For 30 hours prior to this outburst, relative quiet prevailed with the volcano generally emitting ash-bearing explosions of brown to coffee-brown color and very little incandescent lava. Beginning around 0930, explosions became continuous, and they proceeded to squirt and splatter abundant lava E and N of the active crater (MacKenney Crater).

An initial, higher-intensity phase began before noon, sending a gray ash column 800 m high. Such columns later reached roughly 1.5-2.0 km above the crater. The eruption was described in Spanish in a series of INSIVUMEH reports. Their update at 1400 on 20 May stated that ash with grain sizes up to 3-7 mm fell on Guatemala City (the capital), the center of which lies 30 km N of Pacaya. An update at 1700 told of 2.5 mm of ash at the international airport (La Aurora, ~23 km N of Pacaya) but eruptive plumes at the volcano had dropped to 200-300 m in height. The ash at the airport was described as 0.5-1.5 mm across and dark in color. About 1 mm of ash fell in part of the capital.

Although ash fell in the capital mainly during 1100-1700 on 20 May, the amount falling diminished around 1530, allowing the atmosphere to clear substantially. At 1700, Pacaya sent ash plumes 200-300 m above the summit, more typical of normal behavior at the volcano. At about 2000, the wind carried only very small particles toward the capital. Still, local vegetation tenuously held residual ash that could easily become wind blown. In response to the diminished eruptive vigor, at 2100 on 20 May the alert status was lowered from red to orange.

During the eruption, five lava flows or lobes formed, three oriented towards Cerro Chino. One of these reached 1.5 km in length and ~300-400 m in width. The two other flows proceeded down the N and S flanks for ~600 m. No lavas reached cultivated or populated areas. The eruption disrupted the crater sending blocks 2.5 m in diameter up to 1 km downslope (to "la meseta"). Ash thicknesses of 20-30 cm were reported in villages approximately 2-4 km from the volcano (including San Francisco de Sales, Calderas, Mesillas Altas, and El Bejucal).

Verbal conversations with INSIVUMEH's Eddie Sanchez indicated that ash fell on Guatemala City during 1100-1700 on 20 May. Also, volcanic bombs with masses up to 7 kg landed near Pacaya's summit (on Cerro Chino). They also learned that lapilli fell in a village a few kilometers away on Pacaya's flanks (San Francisco de Sales). Otoniel Mat¡as said the 20 May event prompted 252 people to evacuate local settlements. Authorities had several cautions for people in areas of ash fall: 1) drive at speeds below 30 km/hour; 2) avoid the use of their vehicle's windshield wipers because the ash would scratch glass; and 3) cover cisterns even though the ash was non-toxic.

Seismicity during the event on 20 May left records with amplitudes of 5-17 mm peak-to-peak and maintained RSAM values of 870 counts over 10-minute intervals.

The INSIVUMEH report at 1000 on 21 May described a substantial decrease in eruptive vigor but explosions still sent gray, ash-bearing plumes to ~800 m above the summit. A N wind with speeds of 12-17 km/hour carried ash to El Chupadero and El Caracol, spots located 2-2.5 km from the crater. This same morning, the longest lava flow had reached 1.1 km in length.

Other consequences. The previously mentioned aircraft was a commercial jet that was on final approach to the airport when it entered an ash cloud. A few seconds prior to landing, a Pacaya discharge burst forth propelling rocks into the air. Impact with these rocks damaged the pilot's forward windows (captain and first officer) but the landing was completed without further complications. Repairs and inspection were carried out over a 3-day interval; both the airline's technicians and a manufacturer's representative inspected the plane and found the engines undamaged.

At the airport, ash was removed by mechanical means (and to some extent thanks to rainfall) on 21, 22, and 23 May, returning to full service on Sunday 24 May. Rainfall during 22-24 May was never heavy but it apparently did much to wash the ash away. Not surprisingly, the ash clogged storm drains in the capital forcing crews to clean much of the 3-4 million tons of ash mechanically.

A news report in La Naci¢n stated the National Coffee Association computed that the 20 May eruption caused "some $75 million in losses in the coffee harvest." This was the third time in recent history that Guatemala City had been ash choked: the two previous times, 1932 and 1974 were due to eruptions at Fuego, a stratovolcano that sits along the volcanic front roughly 30 km W of Pacaya.

Later activity. Other outbursts occurred during June. One on 14 June was somewhat weaker than the 20 May event; nevertheless it disrupted the crater's geometry and formed a distinct spatter cone. This outburst took place at 1045, exhaling for 10 minutes in conditions of little or no wind, sending brightly incandescent material to 1 km. The material fell harmlessly back on the crater area.

This type of comparatively short eruptive interval had been rare at Pacaya until recently; previous pulses were typically weaker and continued longer, often for 2-7 hours. The short blasts seen recently were thought to be related to water saturation of the ground associated with a wet rainy season; presumably, more groundwater has been driven toward the magma. The situation became difficult from a civil-defense policy perspective since these short, forceful pulses were typically unpredictable and could create conditions requiring rapid response in flank settlements.

Aviation reports. The NOAA/NESDIS Satellite Analysis Branch (SAB) produced tens of reports on Pacaya's mid-May and early June eruptions. For many of their reports before and well after the 20 May eruption, GOES-8 infrared, and multi spectral imagery did not indicate an ash plume, but channel 2 data often revealed a small hot spot. In accord with the rather sudden emergence of the eruption, no ash was detected during clear weather on GOES-8 visible, infrared, or multi spectral imagery through 1645 GMT on 20 May.

Hours later SAB reported a substantial ash cloud; it appeared in GOES-8 imagery taken at 1900 GMT on 20 May (table 2, first entry). Their same report noted that a sounding from Belize (station 78583) had yielded an estimated plume height of about 9-11 km altitude. The cloud extended 140 km NNE from the summit, reached a width of 46 km, and advanced NNE at about 120 km/hour. Table 2 shows a sample of some noteworthy reports posted during portions of 20-22 May.

Table 2. Several of the reports on Pacaya and its ash clouds during parts of 20-22 May put out by the NOAA/NESDIS Satellite Analysis Branch (SAB). Stated times are GMT. Courtesy of SAB.

Report Numbers and Issue Time (GMT)
Observations
98-018 2125 on 20 May
Ash cloud of dimensions and velocity discussed in text
(extending to the point 16°N, 90°W).
98-019 0330 on 21 May
The plume extended NE across Central America and into W
Gulf of Mexico (from 19°N, 88°W to 21°N, 86W ) and was 55
km wide. Plume height was about 9-12 km.
98-020 0915 on 21 May
GOES-8 multi spectral imagery did not show any plume from
the earlier eruption at 1900 GMT on May 20. The plume moved
NW across Central America into the W Caribbean and
dissipated as it approached W Cuba.
98-021 1535 on 21 May
Surface observations at 1500 GMT indicated Pacaya in
eruption. No plume visible on GOES-8.
98-022 1745 on 21 May
Pacaya erupted through 1700. Although ash moved NE at about
50 knots, newly erupted ash was not discerned on GOES-8
imagery. As best as could be determined, the SIGMET issued
earlier by Santo Domingo for ash to 10.4 km spread over
large portions of the N Carribean was the result of ash
resulting from the 1900 GMT 20 May Pacaya eruption. GOES-8
satellite imagery through 1645 GMT failed to reveal
discernible ash in the Caribbean but the presence of thin
diffuse ash could not be discounted.
98-023 0530 on 22 May
Surface observations indicated an eruption from 1500 GMT on
21 May to 0530 GMT on 22 May with uncertain amounts of ash
ejecting. Highly different wind velocities at different
altitudes. Weather clouds obscured the satellite view of
the eruption.
98-026 1430 on 22 May
Surface observations repeatedly indicated that the eruption
continued on 22 May during 0530-1300 GMT but no surface
observations were reported during 0900-1200 GMT. Pilot
reports around indicated ash near 9.1 km in the central
Gulf of Mexico (near 24.3°N, 86.8°W). At some time during
1400-1430 GMT one pilot reported descending to an altitude
of 8.2 km from 8.8 km to avoid volcanic ash. Another pilot
reported no problem while flying at 10 km in the same area.
At some time during 1415-1430 GMT a pilot located over
23.5°N, 86.8 observed a gray layer and smoky smell while
flying at 8.8 km altitude. The pilot could not distinguish
between volcanic ash and smoke. A velocity for ash moving
over the central Gulf of Mexico was estimated based on
upper air data from Key West, Florida: 55-65 km/hour
directed ENE.

Fires, El Nino, and smoky atmospheric conditions. During May and early June INSIVUMEH reported intervals of heavy rains and fog around Pacaya. Satellite data, now available on the web (SSEC, 1998 ), also revealed intervals of cloud cover. Despite this rain at Pacaya, thousands of fires remained burning throughout the region, ~40% of them located in the Petén, an area hundreds of kilometers to the N. These fires and associated atmospheric conditions warrant further discussion as they link to both public safety and the interest in understanding the Pacaya's contribution to the atmosphere.

According to news reports, smoke from forest fires burning out of control added to the airborne ash from 300-m-tall eruption columns during 15-18 May had caused breathing problems as far away as Houston, Texas. In addition, at least one news report said that reduced visibility had made airplane landing possible only through the use of instrument guidance in Guatemala City; Honduras was forced to close its two largest airports.

What follows came from a report by the U.S. Agency for International Development (21 May 1998). The report noted that since January, more than 10,650 fires have burned some 1,200 square miles [3,108 km2] in Mexico, an area nearly the size of the state of Rhode Island. As of 21 May, approximately 277 wildfires still raged throughout Guatemala.

"During 1998, Mexico and the entire Central American region have been affected by drought exacerbated by El Nino conditions. The drought has aggravated the effects of slash and burn agricultural practices in forest and grassland areas, leaving thousands of fires burning out of control. Tropical forest, usually too humid to burn, has become extremely vulnerable to fire. In addition to making the land more arid and therefore more flammable, the droughts have eliminated the cleaning effect that rains usually have on the region's air. The ground cover burning may be the driest ever recorded in this century, which has resulted in large quantities of smoke being emitted into the atmosphere. The fires have burned more than one million acres [>4,050 km2] and severely affected visibility and air quality in Mexico, Guatemala, Nicaragua, Honduras, El Salvador, and Costa Rica. In Guatemala, Honduras, and Nicaragua, an estimated 2,146 square miles [5,558 km2] have burned. The smoke from these fires also has entered the southern and Midwestern United States prompting local warnings for residents with respiratory conditions to limit their outdoor activities."

"According to NASA, more than 2,000 fires are currently raging in Guatemala. The U.S. Embassy in Guatemala City reported on May 19 that the fires are intensifying and are threatening human populations. The "red alert" on air quality declared by the Government of Guatemala (GOG) on May 15 remains in effect. Air quality monitors report that total suspended particulate levels in Guatemala City averaged 600 milligrams per cubic meter during the first two weeks of May, three times the World Health Organization maximum level. Since then, air quality has worsened significantly. According to the Embassy, the GOG's Ministry of Health's local health centers have found significant increases in respiratory ailments. The Ministry of Health says that the most heavily affected areas are Petén, Alta Verapaz, Baja Verapaz, and areas of Huehuetenango and Quiche. In Ixcan, 80% of the population are suffering from respiratory-related ailments, eye irritation, vomiting, and headaches, according to local community leaders."

Explosive activity resumed on 2 January 1999 at Pacaya for the first time since the end of a major eruptive episode on 19 September 1998. Current activity has consisted of small explosions that ejected ash without incandescent material. Beginning on 8 January, the number of explosions increased from 100-200/day to more than 400/day, reaching a peak of ~ 550 on 21 January (figure 19). Explosion counts declined to ~200/day by the end of the month. Volcanologists from INSIVUMEH and the Smithsonian Institution observed frequent small ash eruptions during a 1 February visit. The explosions were not accompanied by detonations, and produced billowing gray-to-brown ash columns that rose ~100 m above the vent. They observed that two vents produced explosions; the largest explosions originated from the westernmost and lower of two vents in the breached crater. Intense fumarolic activity occurred from the inclined floor of the summit crater, its rim, and the outer flanks.

Figure 19. Daily explosion counts at Pacaya during January 1999. Courtesy of INSIVUMEH.

Significant changes to the morphology of MacKenney cone had occurred since a strong explosive eruption on 18-19 September 1998. That eruption left a major breach 20-25 m wide that extended SW. By the time of the 1 February visit, erosion had widened the breach to 70-80 m. At its head, the breach had nearly vertical walls more than 50 m deep, and formed a gully that extended more than 1 km down to ~1,800 m elevation. The NE side of the crater was also notched, but not nearly as deeply. Fractures and down-dropped blocks of summit agglutinate material along the crater rim also showed this SW-NE orientation in line with the location of two flank vents active during September 1998. The breach gives MacKenney cone a twin-peaked appearance when viewed from the W flank (figure 20). The present form of the crater increases the possibility of future eruptive or collapse events being directed toward the W-flank village of El Patrocinio (figure 21).

Figure 20. A prominent gully extends more than 1 km down the SW flank of Pacaya from the twin-peaked summit of MacKenney cone, 1 February 1999. The dark lava flow at the lower right was one of two emplaced from flank vents at the end of the 18-19 September 1998 eruption. Photograph courtesy of Lee Siebert.

The accumulation of spatter and ejecta from the September 1998 explosions had built MacKenney cone to a height about 30-35 m above an older cone immediately SE of MacKenney crater. The older cone, the previous vantage point for observing explosive activity from Pacaya, had itself grown about 10 m in the past decade from the accumulation of ejecta from MacKenney crater. The height of MacKenney cone now exceeds that of Cerro Grande, a vegetated ~2,560-m-high prehistorical cone of Pacaya located 2 km NE of MacKenney.

September 1998 eruption. A major explosive and effusive eruption took place on 18-19 September (table 3). During the first 17 hours of the eruption, a 1.2-km-long lava flow descended WNW into the caldera moat and down the flank of the volcano to the Montanas las Granadillas area SW of Cerro Chino. From 1700-2200 an explosive eruption ejected ash columns to 5 km above the crater, producing ashfall to the SW and NNW. Fine ashfall caused the closing of the international airport in Guatemala City for 35 hours. About 1 m of volcanic bombs were deposited on the caldera rim. Pyroclastic avalanches of incandescent ejecta mantled the upper half of the cone. One 3-m-wide impact crater was formed at the base of the lava flow near El Patrocinio, and 1-m-wide impact craters were found as far as 5 km from the vent. During the final explosive phase, the SW rim of MacKenney crater collapsed, forming a debris avalanche that traveled 2 km down the SW flank to ~1,500 m elevation. Coarse blocks littered the surface of the deposit, whose light color contrasted with that of adjacent dark-colored lava flows.

Table 3. Summary of major eruptive events at Pacaya volcano from January 1987 to September 1998.

Date Description of Volcanic Activity
21 Jan 1987 Ash fell over areas of the villages of
Amatitlan and Santa Elena Barillas. The
villages of El Caracol and El Patrocinio
were evacuated.
25 Jan 1987 10-15 cm of ash fell over El Caracol, El
Rodeo, and in part over El Patrocinio.
14 Jun 1987 Lava flow reached 2.5 km SW; 600 people
evacuated.
7-11 Mar 1989 Two lava flows threatened to reach El
Patrocinio and El Rodeo. A third lava flow
traveled 3 km on the W flank.
2 Apr 1990 A 4-hour-long eruption deposited 10 cm of ash
in El Patrocinio and El Caracol.
15 Sep 1990 Moderate intensity eruption caused a moderate
ash fall over El Patrocinio.
5 Mar 1991 Minor ashfall in El Caracol and El Patrocinio.
6,14,16 Continuing eruptive activity destroyed the
Jun 1991 active crater (MacKenney).
8,12,14,15 Moderate intensity eruption; minor ashfall over
Jul 1991 El Caracol (3 km from the crater).
27 Jul 1991 An eruption caused a 26-cm-thick ash layer to
be deposited over El Caracol and El
Patrocinio, 1.5 cm in Escuintla, and a thin
layer in Santa Lucia Cotzumalguapa.
1 Aug 1991 A 3,000-m-high column caused ashfall over
Barbarena and Cuilapa.
10 Jan 1993 Collapse in the active crater sent a glowing
avalanche to the side of El Caracol. The
post-collapse eruption column drifted toward
Santa Lucia Barillas. The acidity of the ash
damaged vegetation in the region.
21 Sep 1993 4-hour eruption caused a minor ashfall over El
Caracol.
16 Mar 1994 Eruption lasted until midnight and had an
incandescent lava fountain 300 m high. Most
of the ash fell on the volcano's flanks.
15 Oct 1994 Phreatomagmatic explosion; acid ashfall damaged
vegetation in Santa Elena Barillas and Los
Llanos. Population was affected by pulmonary
and respiratory problems.
7 Apr 1995 A lahar completely covered a house and killed a
little girl in Los Rios. The inhabitants were
evacuated as 25-35-cm-thick volcanic sand was
deposited over the village. As a result of a
hazard study, many villagers had been
previously evacuated.
1-7 Jun 1995 A debris avalanche caused by collapse of the W
crater rim destroyed a radio station and
partially burned the vegetation of Cerro
Chino in a 4-km2 area.
7 Jun 1995 Lahars moving as a dense, dough-like mass, cut
roads and wiped away a bridge. Consequently
many families in El Patrocinio and Los Rios
were evacuated and later part of the
population was relocated in La Colima.
17 Sep 1995 A 1-km-high column from a phreatomagmatic
explosion deposited 3 cm of fine ash in Santa
Elena Barillas and a fine veil of volcanic
dust in Barbarena and near Cuilapa.
11 Oct 1996 At dawn the eruption produced a sustained lava
fountain 500-700 m high and lava flows as
long as 1.5 km on the SE flank. The 35 km/h
wind with blasts at 45 km/h caused a fine ash
fall as far as Puerto San Jose, 60 km to the
S on the Pacific Ocean.
11 Nov 1996 A 9-hour-long eruption produced a 2-km long
lava flow and deposited 7-12 cm of ash near
El Caracol and Finca El Rabon. El Rodeo
received a 2-3 cm thick blanket of ash. It
was necessary to evacuate the population of
El Caracol, El Rodeo, and some women and
children of El Patrocinio.
20 May 1998 A 5-hour eruption produced a 4-km-high ash
column. S wind caused ashfall in the capital
City, Ciudad de Guatemala (2 mm in the N and
4 mm in the S areas of the city). La Aurora
International airport was closed for three
days. Incandescent bombs and hot blocks
ignited trees in the mountainous areas of
Cerro Grande, 2 km NNE of MacKenney crater.
254 people were evacuated from San Francisco
de Sales, El Cedro, and El Pepinal. Two
people were injured by falling scoriaceous
bombs in S.F. de Sales.
14 Jun 1998 A moderate eruption began at 0600 and lasted
until 1900. An incandescent lava fountain was
oscillating between 150 and 400 m high. A
large ash column (600-800 m high) was blown
to the S and produced scoriaceous ashfall in
El Caracol. There was no need to evacuate.
Condensation of atmospheric humidity due to
the heat fed a cloud that reached 1,500-1,700
m in height. The Unidad Coordinadora Deptal
de Escuintla del Ministerio de Agricoltura,
Ganaderia y Alimentacion reported the loss of
Q70,000 (US $10,000) from partial destruction
of coffee, corn, and bean crops, and for
purchase of food for livestock. Aircraft
reported ash at 5,500 m.
18 Jun 1998 A 10-minute explosion at 1045 caused the
ejection of semi-incandescent blocks (>= 35
cm) over all the volcano flanks. Then, 20
minutes later, fine ash lightly fell over the
city of San Vincente Pacaya.
18 Sep 1998 The main eruption had one effusive and one
explosive phase. The first lasted 17 hours,
producing a 1,200-m-long tongue of lava that
emerged from the WNW rim of the active crater
and then deviated to the Montanas las
Granadillas area SW of Cerro Chino. The
second phase occurred from 1700 to 2200
hours. It expelled an ash column that reached
5,000 m altitude and produced ash and lapilli
fall to the SW and NNW.A very thin film of
fine ash (~ 1 mm) caused the La Aurora
International airport to be closed again for
35 hours, after which it reopened with
restrictions. Three lava flows accompanied
the explosive phase; the first one, 400 m
long, went WNW and reached the base of the
cone. There it joined the second flow (from
the N flank). The third lava flow departed
from the second flow and went to the S toward
El Caracol. During the proximal explosive
phase the SW rim of the MacKenney crater
collapsed, causing a debris avalanche 2 km
long, and a cloud of hot ash and gases that
burned vegetation in the distal reaches.

Several lava flows accompanied the explosive activity (figure 22). The longest of these traveled ~4 km from a notch in the NE crater rim. The flow initially descended northward into the caldera moat where it was deflected by the caldera wall, flowed across the moat, and then down the SW flank to 1,760 m elevation before diverging around a small kipuka and scorching trees at its northern margin below Cerro Chino. Much of the caldera moat was covered by lava flows of the September eruption, and the prominent 1984 spatter cone low on the N flank was nearly buried.

Figure 22. Photograph of the lava flow (foreground) that descended from Pacaya's caldera moat down the W flank. This flow and the two dark lobes above it originated from MacKenney cone during the 18-19 September 1998 eruption. Light-colored tephra deposits between the flows mantle previous lava flows. Photograph taken on 1 February 1999. Courtesy of Paul Kimberly, SI.

At the end of the eruption, two small lava flows took place from flank vents on opposite sides of the cone. A vent on the upper NE flank at ~2,450 m elevation produced a short lava flow that reached the caldera moat. A vent on the lower SW flank at ~1,800 m elevation (figure 22) produced a short lava flow that divided into two lobes, one traveling to the SW and the other to the south.

Summary of 1987-98 activity. Routine explosive activity characteristic of Pacaya occurred through much of the period from 1987 to the present but is not listed in table 1. Strong explosive eruptions in January 1987 and June 1991 destroyed the upper part of MacKenney cone, deepening and widening the crater, after which renewed eruptions reconstructed the cone. Major eruptions on 7 and 14 June 1995 destroyed the WNW side of the crater, leaving two notches at the summit. Debris from the 7 June collapse slammed into the caldera wall at Cerro Chino, 1 km NW of the summit, and produced a secondary hot cloud that swept over Cerro Chino, destroyed a radio antenna, and affected houses within 2 km of the active vent. The shockwave threw INSIVUMEH observer Pastor Alfaro down a slope, fracturing his leg. The 7 June event produced a 2.5-km-high plume. The second collapse on 14 June produced an avalanche that traveled SW toward El Rodeo and was accompanied by a 4-km-high plume. Lava flows subsequently traveled 2 km. Figure 23 shows RSAM plots for 1995-98.

A strong explosive eruption on 20 May 1998 produced a 4-km-high ash column. Incandescent bombs burned trees on the SSW flank of Cerro Grande, 2 km N of the crater, and scoria fall damaged vegetation and crops. Two persons in the settlement of San Francisco de Sales, 2.5 km NE of the crater, were injured by falling scoria blocks. The ash plume was primarily blown to the NE, with a lesser plume to the SW (figure 24). Ash fell from 1300-1600 in the villages and towns within 5 km of the volcano. During 1400-1830 ash fell in the capital city of Guatemala, causing closure of the international airport. Ashfall covered an area of 800 km2, and had an estimated volume of ~2.3 x 106 m3. The eruption caused the evacuation of 254 residents from surrounding villages to the town of San Vicente de Pacaya. Lava flows during the 20 May eruption traveled down the N, W, and SW flanks and had a volume of 6.3 x 105 m3.

An INSIVUMEH report noted that Pacaya, which erupts frequently, had been relatively quiet through at least 1 September 1999. At that time its behavior was fumarolic only and tremor registered sporadically.

The rest of this report covers the interval from late December to 16 January. Anomalously large explosions took place during the days preceding 23 December, when Strombolian eruptions sent material 300-500 m high. The explosions originated in a narrow depression 50-75 m long on the upper SW flank, facing the city of Patrocinio. The activity was observed from points on the S coast. Shallow explosions created earthquakes in and around the volcano.

Atmospheric conditions remained relatively stable; a clear, calm wind blew from the N at ~35 km/hour. On 29 December weak explosions deposited rocks and fine ash over the edifice, and a constant fumarole emitted white and blue colored gases. The explosions that day created a plume 25 m high. A lava flow extended 75 m from the crater rim toward the SW, in the direction of Patrocinio. Tremor and explosions also occurred.

In January 2000, Pacaya remained restless. Otto Garcia reported that on 14 January a period of Strombolian activity began with lava flows and small explosions. Then, on 16 January, a comparatively violent explosion took place. Photographs taken from the NW (figures 25-28) showed high tephra ejections, a dark plume, and lava flows. According to NOAA reports received by Garcia, the tephra-bearing plume reached ~8-km altitude after the eruption climaxed around 2245. Winds spread tephra S and SE to the provinces of Escuintala, Siquninala, and Santa Lucia Cotz, and as far as 50 km S of the volcano. Lava fountains that rose 500-750 m above the cone were visible from Guatemala City, ~35 km N; the vigorous outburst lasted ~4 hours. In nearby villages 1,500 residents were evacuated.

Figure 25. A powerful incandescent lava fountain rises above the summit of MacKenney cone at Pacaya on 16 January 2000. This view from the NW shows the forested flank vent Cerro Chino to the left. A dense ash column rises and is blown SW. A more vigorous phase of the eruption apparently occurred later, after dark. Courtesy of Manolo Barillas (CONRED).

Figure 26. A daytime scene of the 16-17 January Pacaya eruption. The view is from the NW. Courtesy of Manolo Barillas (CONRED).

Figure 27. Pacaya's fountaining seen on the night of 16-17 January 2000. The view is from the NW. Courtesy of Manolo Barillas (CONRED).

Figure 28. Dense, billowing ash clouds from Pacaya on 16 January 2000. Cars along the road between the towns of Amatitlán and Esquintla are in the foreground. Courtesy of Manolo Barillas (CONRED).

The eruption was photogenic and appeared in numerous media reports. A CNN news report stated that by 17 January Eddie Sanchez of INSIVUMEH estimated that the lava flow had advanced 900 m down the mountain.

Tourist's photos and impressions. On the afternoon of 16 January guided tourist groups were visiting the volcano; some had ascended to an overlook when unusually energetic explosions occurred. One tourist, Gene Weast, wrote of his experience during the explosion in a brief letter that appeared on a website along with documenting photographs, including figure 29.

During the hike up the volcano, the guide cautioned the group to keep their distance from fresh lava, since such material had become larger and hotter in the last few days. Every 10 seconds an explosion from the summit sent rocks and lava 40-100 m high. As the sun began to set the explosions suddenly became louder, and chunks of lava (volcanic bombs) were thrown 0.5-1 km into the sky. Soon after the large explosions commenced, these bombs began to fall on the sides of the mountain. All of the guides and tourists ran back down the volcano and managed to escape the explosions safely.

Figure 29. A photo of the violent 16 January 2000 explosion at Pacaya taken in the late afternoon with an 80 mm lens as the photographer stood on the rim of La Meseta, ~500 m from the summit vent. A
Strombolian explosion ejected a spray of incandescent lava hundreds of meters into the air; the stream was framed by billows of dark, gray plumes that continued to rise above it. Some erupted material escaped through a notch in the
MacKenney cone and formed a small lava flow that bifurcated in the lower part of the photograph. This flow headed toward the moat between the MacKenney cone and La Meseta rim. Courtesy of Gene Weast.

Map of new lava flows, satellite data, and perspective on the 16 January eruption

A vigorous new phase of explosive and effusive activity began at Pacaya in late 1999 (BGVN 24:12). The eruption began with Strombolian activity on 23 December 1999 and, by 16 January 2000, built a ~50-m-high cinder cone within the summit crater. On 4 January the lava had started to flow beyond the summit crater and, by 10 January, the lava extended 1 km down the SW flank. Subsequently, the summit crater fed a second lava flow that descended the 30-40° slopes along the N flank of Pacaya's central cone (figure 30). At the base of the cone, the lava channel bifurcated and the outer crater wall deflected the flows NW. Along its medial section, the southernmost channel was 5-7 m deep and 7-10 m wide. By 2130 on 14 January this portion of the flow had formed a tube. After exiting the channel and tube system, the lava formed a pond in the basin between the base of the cone and outer crater wall building a 200-m wide compound aa lava field ~1 km long. This field overlaid most of the lava field formed by the September 1998 eruption.

At 1615 on 16 January the eruption increased in intensity and changed from Strombolian to fountaining. These fountains fed ash plumes, which were observed on GOES images, and the cinder cone built earlier was destroyed during the first 5 minutes of the high intensity activity. Fountaining initially reached heights of 800 m but diminished to 300 m by 1830. Fallout from the fountains fed near-constant nuées ardentes and a lava flow that extended 600-700 m to the SW (figure 3). Within the summit crater lava fountaining appeared from about four vents, or possibly a fissure, with the highest and most vigorous fountains issuing from the southernmost sources. Fountains from these southernmost sources projected obliquely to the S. Strombolian activity returned around 2030.

The 16 January episode was one of the most spectacular at Pacaya in all of its current 35-year-long history of eruption. While the population of Guatemala City watched the fountaining, the fall of over 30 cm of tephra in the area S of the vent forced over 1,000 people to leave that area and closed the Aurora Airport that evening.

Activity in the morning of 19 January was characterized by low-level degassing but at 1325 an ash plume rose ~5 km above summit level. Ballistic block fall occurred during the first minute and a maximum temperatures of 97°C was measured at the base of the column a few minutes later. The plume spread mainly to the S, but wind shear caused spreading of the plume to the N and W. The intensity of ash emission began to wane and by 1415 the plume height had declined to 200-300 m and the ash plume temperature decreased to 24-35°C. Persistent ash emission continued from at least two sources within the summit crater throughout the afternoon with frequent larger pulses causing the plume to increase in height and temperature. Persistent ash emission continued to heights of ~50 m, with occasional puffs pushing the plume to ~100 m, causing higher temperatures and ash drifting 3-6 km to the S. Seismic records show that the high intensity portion of this eruption lasted 8 minutes followed by a 30-minute-long hiatus and then 7 to 8 pulses each lasting between 45 seconds and 4 minutes.

Seismic records indicated a renewal of activity in the late afternoon of 22 January. GOES hot spot monitoring tool (http://hotspot.higp.hawaii.edu/) indicated hot-spot activity beginning between 1725 and 1745 and relating to intracrater activity. A Landsat-7 Enhanced Thematic Mapper (ETM+) scene acquired at ~1030 on 23 January shows an intense thermal anomaly within the summit crater, a point source 180-240 m in diameter. The absence of an elongated anomaly extending from this source indicates that flow activity had not begun at that time.

By 24 January, Strombolian eruptions fed a persistent, diffuse plume of fine ash that rose ~1.5 km above the summit. Later observations on the same day showed vigorous, near-continuous Strombolian activity with explosions occurring at a rate of 19-28 events per minute and throwing incandescent bombs as large as 1-2 m in diameter and occasional ribbon spatter to heights of about 300 m. At nightfall observers from Guatemala City could see a continuous glow from the crater. Bombs from at least two vents landed within a few hundred meters from the vents on the N, S and W flanks; the ejecta on the S causing incandescent avalanches within 200-300 m of the crater as bombs tumbled down the steep slopes of the summit cone. Lava flow extended southward for 400-500 m, and turned SSE to SE at the base of the cone. For the first 400-500 m, the flow was mainly contained in a 5- to 10-m-wide channel with two heavily crusted sections, but at the break of slope of the cone an incandescent aa flow widened to ~200 m. The flow front contained three lobes, each 30-70 m wide and the longest of which advanced ~ 1 km from the vent (figure 3).

Although a persistent ash plume was observed from Guatemala City the following day ( 25 January), the plume had disappeared by 0900. A rapid decline in GOES-recorded hot spot radiance between 2143 on 24 January and 0056 on 25 January indicated that lava had ceased to flow.

At 1815 on the evening of 29 February, a second major eruption began. An ash column rose 2 km into the sky and then, at 2140, a column of erupting lava rose to ~700 m above the summit. Wind blew the ashes to the SSW, falling on the towns of Escuintla and Siquinala. The National Disaster network declared a red alert and surrounding communities were evacuated. A hot spot appeared on the GOES image around 2045 and a peak of activity was reached around 2200 which diminished sharply thereafter. However, by 1030 on 1 March, the hot spot intensity was still fairly high, albeit lower than the peak the previous evening.

On 1 March Andy Harris and co-workers in the HIGP/SOEST group noted a "big flash" in the GOES data depicting Pacaya. This appeared to develop from a hot spot that began around 2045 on 29 February (0245 on 1 March GMT), possibly as early as 2015. Peak hot spot activity (and the flash) was reached around 2202, and an ash cloud tracking NE (which extended seven to nine GOES pixels during 15 minutes) was apparent in imagery at that time. After 2202, the hot spot intensity fell sharply but as of 1032 on 1 March it still remained.

Harris also noted that their system issued an automated email notice for this event at 0424 GMT, just 22 minutes after acquiring the image. Since their system alerted them to the hot spot a few hours prior to the main flash and ash emission, this enabled them to watch as activity escalated.

Activity at Pacaya was at a low level during 21-23 January 2001 based on field observations. Activity was characterized by persistent degassing only from a vent on the floor of the MacKenney cone crater. During the afternoon of 22 January, a low-resonance rumbling was noted within 100 m of the crater rim and continuous tremor was felt at the crater rim. Such activity had not been apparent during the previous afternoon.

On 22 February, according to a report from INSIVUMEH, a local seismometer recorded >600 tremors/day, up from <100 tremors/day in earlier weeks. At the same time, gaseous emission had increased from ~253 metric tons/day (equivalent to the SI unit megagrams per day, Mg/d) to ~550 metric tons/day and there was visible magma in the crater.

The SO2 flux was measured by COSPEC along the highway from Guatemala to Escuintla on 1, 15, and 28 February and on 8 March. The respective flux values in metric tons/day were 951, 1,740, 1,448, and 1,673. These flux rates suggested that although lava outpouring was almost nonexistent, degassing of SO2 was quite high. The average gas emission rate for the 20-year period of 1973-93 at Pacaya was estimated by Andres and others (1993) to be 260 metric tons/day.

At least two commercial aircraft flew through airborne ash near the Guatemala City airport on 21 May 1999. An eruption from Fuego that day was the first at that volcano since 1987 (BGVN 24:04). Although the aviator's reports attributed the ash from this encounter to Fuego, their aircraft intersected multiple ash plumes in widely different locations, and thus they may also have crossed plumes from Pacaya. The most likely plume near the southern approach to the airport (La Aurora, ~23 km N of Pacaya; ~40 km NE of Fuego) is from the almost constantly active Pacaya. In contrast, Fuego lies 32 km W of Pacaya and was the likely source of a plume intersected later during the flight, at higher altitude, and for much longer duration.

During the encounters the ~100 or more people on board the two aircraft, and many more on the ground, were at risk. Both encounters seriously damaged the aircraft but ended in safe landings without reported injuries. Volcanic ash can cause jet-engine failure, which creates a hazard not only to passengers, but to people on the ground as well. The risk in this situation was amplified by the airport's proximity to urban Guatemala City (population, >1.1 million).

This example leads to two conclusions discussed further below. First, ash avoidance methodology needs further refinement. Second, there exists an apparent bias towards under-reporting of aircraft-ash encounters, which could short-change their cost to air carriers, their perceived risk, and their funding allocation.

The original report of an aircraft-ash encounter was brought to our attention by Captain Edward Miller of the Air Line Pilots Association and provided on the condition that the air carrier remain anonymous. Bulletin editors also wish to acknowledge conversations with several additional anonymous contacts (commercial pilots). This case occurred in Guatemala; however, analogous situations exist at many airports adjacent to volcanoes. Modest eruptions from nearby volcanoes may be uncertain or difficult to see; they may be hard to detect or characterize; yet they still may yield mobile ash plumes carried by complex local winds to confront air traffic (Salinas, 2001; Hefter, 1998; Casadevall, 1994).

Although in most Bulletin reports we favor the use of local time to emphasize the reference frame of people on the scene, most of the source material (satellite and aviation) for this report refer to UTC (Coordinated Universal Time). UTC is the time on the Greenwich meridian (longitude zero), formerly GMT, a term which has fallen out of use. Accordingly, there are cases in this report that lack conversion to local time. Local time in Guatemala is 6 hours behind UTC.

Activity at Fuego and Pacaya. Fuego erupted on 21 May 1999 sending ash to the S, SE, and SW and ultimately dropping up to 40 cm of ash on local settlements (BGVN 24:04). The eruption occurred at 1800 local time (in terms of UTC, at 0000 the next day). Three hours later the eruption decreased and the Aeronautica Civil recommended that planes go no closer to Fuego than 40 km. One hour after that (at 2200 local, 0400 UTC), the atmospheric ash had settled, and Aeronautica Civil recommended flying no closer to Fuego than 15 km.

Bulletin reports for Pacaya in mid-1999 suggested relative calm, in harmony with the observation that it had chiefly been fuming. However, Pacaya is well-known for Strombolian outbursts. It lies directly in-line and only 23 km S of the N10°E-oriented airstrip. As is common for commercial aircraft there, the approaches described below passed very close to Pacaya. Pacaya typically has lower and smaller eruptions than Fuego, but because it lies so close to the southern approach to the airport, Pacaya's ash plumes easily enter the path of landing aircraft. The situation was particularly complex during this eruption because Fuego's ash was reported on cities that lie below the flight path as well as on the Pacaya's flanks.

Pilot's report of aircraft-ash encounter. What follows was taken from one flight crew's description of events and from later reports and dialog on the topic. In order to preserve the confidentiality of the airlines, the precise times of events on 21 May 1999 have been omitted. Pilots reported clear visibility and light wind, with both the capital and the airport in sight. They maneuvered the aircraft for a final approach from the S. Pilots were advised by air traffic control about erupted particulate ("volcanic sand") about 24 km (15 miles) SE of the airport, well away from their projected path to Runway 01. In addition, based on winds they detected as they neared the airport, the pilots concluded that the erupted particulate to the SE was downwind from their projected flight path to the airport. (In retrospect, this conclusion appears tenuous considering the possibility of either a fresh injection of ash from Fuego, or lingering ash in the trailing portion of the plume that lay to the SE.)

At ~32 km (20 miles) distance from the runway, the pilots maneuvered the plane through ~3,000 m (9,700 feet) altitude on a final approach to the airport with the plane's flaps partially extended (at ~15°, which slowed their aircraft to ~260 km/hour (160 mph), its auxiliary power unit (a small jet engine) on, and its landing gear down. Around this point in their descent, pilots saw bright yellow sparks through the windshield lasting a few seconds, a display unlike static electricity, but rather like that from a grinding wheel. This occurred again at 2,500 m (8,200 feet) altitude, but was more intense yet intermittent.

At this time, air-traffic control announced to the pilots that the aircraft landing in front of them had encountered volcanic particulate; they instructed the pilots to abort their landing and climb to 3,350 m (11,000 feet) altitude. Discussions of options and ash avoidance ensued between pilots and air-traffic control; ash had by this time accumulated on the runway, further complicating landing, even for approaches in the opposite direction. The pilots retracted the landing gear, accelerated to ~410 km/hour (~250 mph), began to climb, and after some discussion with air-traffic control, held on a course NE of the airport. They were subsequently cleared to climb to 6,700 m (22,000 feet) altitude and advised to proceed to an alternate airfield.

During the climb, at ~5,800 m (19,000 feet) altitude en-route to the alternate airport, the aircraft encountered volcanic particulate for 10 minutes. Within that interval the plane spent 2 minutes during ascent engulfed in denser and heavier particulate. During that period, window arcing was constant and the beam of their landing lights revealed a conspicuous cloud of reflecting particles. During ash ingestion the engines's speed lacked noticeable fluctuations. The aircraft exited the plume at about 6,100 m (20,000 feet) altitude and landed without encountering additional ash.

Upon landing, the pilots noticed reduced visibility through abraded windshields. Post-flight examination of the airplane revealed heavy damage, requiring the replacement of the engines (US $2 million each) and auxiliary-power-unit engine. (Note, however, that no deterioration in engine power or performance was noticed during the flight.) Other replaced parts included windshields, a heat exchanger, and coalescer bags. Minor damage was seen on the horizontal stabilizer and wing leading-edges.

Volcanic Ash Advisory Statements. The U.S. National Oceanic and Atmospheric Administration (NOAA), Satellite Services Division website contains two archived statements issued by the Volcanic Ash Advisory Center (VAAC) at Washington, D.C. for Fuego on 21-22 May 1999. The statements, issued to the aviation cummunity to warn of volcanic hazards, are intended for an audience accustomed to special terminology (figures 31 and 32). In the interest of advancing understanding of how volcanological and atmospheric data get transmitted to aviators, we offer brief explanations for many of the terms used (figure 31).

Figure 31. The first archived Volcanic Ash Advisory Statement (VAAS) for the 21 May 1999 eruption at Fuego, Guatemala. The boxes contain added notes to explain some of the basic conventions and specific details seen here. This Statement currently appears on the NOAA Satellite Services Division website (see "Information Contacts," below). Local names are frequently anglicized, dropping all accents and other non-English characters (eg. México would be written MEXICO). Later Advisories adopted the abbreviation "Z" (pronounced 'Zulu' by aviators) for UTC, as in 1300 UTC written as 1300Z.

Figure 32. A later Volcanic Ash Advisory Statement (VAAS) for the 21 May 1999 eruption at Fuego, Guatemala. The statement, which was issued the next day, discloses that the eruption had then stopped and the hazard status was lowered. The statement appears on the NOAA Satellite Services Division website (see "Information Contacts," below).

The Washington VAAC received first notification of the Fuego eruption from a routine surface weather observation from Guatemala City at 2000 local time (0200 UTC) on 22 May. They issued the first Volcanic Ash Advisory Statement a half-hour later (figure 31). Six hours later they issued the second Advisory Statement (figure 32). The Advisories were composed by staff of the Satellite Analysis Branch, one of the two NOAA components forming the Washington VAAC (Streett, 1999; Washington VAAC).

The section "Details of ash cloud" first says that the "surface observation from Guatemala City indicate that the Fuego volcano is in eruption" and that no additional information is available and then briefly describes in words observations that came from satellite imagery. The first sentence, "No eruption . . ." is self-explanatory, but highlights a limitation of the method in use that needs to be emphasized to aviators: an eruption may have occurred but its status is not revealed on the imagery. The sentence, "No eruption could be detected due to thunderstorm cloudiness covering the area around the volcano" is self-explanatory. Less clear is the term convective debris. It does not refer to ash; rather, it refers to remnants of thunderstorms. The gist of these latter two sentences is simply that the thunderstorms that covered the area made it challenging or impossible to see the ash on satellite imagery. Central American thunderstorm clouds typically can reach altitudes of more than 12,000 m (40,000 feet), and can mask or obscure airborne ash residing below that level.

Members of the Washington VAAC commented that this eruption demonstrates key problems that can arise when cloudy conditions prevent satellite detection of ash, foiling a primary mode of analysis. In such cases, they rely on ground observers (including observatories and weather observers), pilot reports, and reports from airlines. Thus, their ability to issue useful information in cloudy conditions depends on the quality of communications with local observers, the Meteorological Watch Office, volcanologists, geophysical observatories, and the aviation community.

The Advisory Statement listed México City weather balloon data acquired 1,050 km NW of Guatemala City. In retrospect, the Washington VAAC noted that they generally avoid using such distant sounding data. If a closer sounding cannot be found, they prefer to use upper-level wind forecasts taken from a numerical weather model. In any case, the scarcity of local sounding data presents a challenge to the realistic analysis of airborne ash.

The outlook section says to "see SIGMETS." SIGMETS are the true warnings to aircraft for SIGnificant METeorological events. They are issued by regional Meteorological Watch Offices (MWOs), in this case the MWO (for Guatemala) is in Tegucigalpa, Honduras. SIGMETS were lacking for this eruption; although the Washington VAAC tried to contact the MWO without response. Another complexity confronted at the VAAC is a lack of a single scale for communicating a volcano's hazard status.

Reporting of aviation ash encounters. In personal communications with Bulletin editors, airline personnel stated that many more encounters have occurred than have yet been tallied in publically accessible literature. In accord with those assertions, the 21 May 1999 encounters are absent from reports compiled by the International Civil Aviation Organization (ICAO, 2001). In that document (Appendix I, table 3A, p. I-12) Fuego fails to appear as a source vent for any aircraft-ash encounter. Pacaya is listed for two encounters, in January 1987 and in May 1998 (BGVN 23:05).

Even though the number of encounters was probably under-represented and thus reflects a minimum, ICAO (2001) notes that the international costs to aviation since 1982 summed to well in excess of $250 million. They noted, "In addition to its potential to cause a major aircraft accident, the economic cost of volcanic ash to international civil aviation is staggering. This involves numerous complete engine changes, engine overhauls, airframe refurbishing . . . aircraft downtime . . . [and] volcanic ash clearance from airports and the damage caused to equipment and buildings on the ground."

The incidents here suggest that there has been a strong bias toward under-reporting aircraft-ash encounters. If this tentative conclusion is correct, it implies consistent understatements of the hazard's magnitude. This, in turn, may have thwarted meaningful analysis of how and whether to proceed with designing more robust hazard-reduction systems. Accordingly, resources that could have been devoted to the problem have not yet been committed (see Gimmestad and others, 2001 for a discussion of a prototype on-board ash-detection instrument).

Communication challenges. While much of the aviation community needs to learn about volcanism rapidly, dependably, and with the aid of the Internet, some observers charged with reporting volcanic-ash hazards in Central and South America lack access to basic communication devices like reliable telephones and fax machines. To reduce the risks, the aviation, meteorological, remote sensing, and volcanological communities need to improve their ability to pass critical information to each other rapidly and precisely. The operational systems related to volcanic ash and aviation must transcend numerous boundaries (eg., languages, infrastructure, funding, governments, agencies, air carriers, pilots, aircraft manufactures, etc.). The systems need to portray complex, dynamic processes such as the rapid rise of an explosive plume, or large-scale ash-cloud movement.

Although the infrastructure for ash avoidance is greater than ever, members of the Washington VAAC have told Bulletin editors that they still depend heavily on people on the scene of the eruption to notify them promptly when eruptions occur. They said that thus far in parts of Central and South America a problem has been the expense of communication (eg., by phone, fax, and Internet). They also said that for the same regions the U.S. meteorological database regularly lacks pilot reports. Though serious, these problems have at least been identified and their solutions would appear to lack great technical or economic barriers.

The pilots involved in the May 1999 encounter recommended that far more emphasis be placed on forecasting and avoiding ash plumes. Other pilots cited the need for fast and accurate communications between those who observe eruptive activity and air traffic control personnel.

Issues like these continue to be an important subject at gatherings on the topic of ash hazards in aviation (Casadevall, 1994; Streett, D., 1999; Washington VAAC, 1999). The Airline Dispatcher's Federation (ADF) will participate in a 7-9 May 2002 conference and workshop: "Operational Implications of Airborne Volcanic Ash: Detection, Avoidance, and Mitigation." The gathering will provide the pilot and dispatcher with insights into volcanic ash, including its characteristics, affects on aircraft, detection/tracking, effective warning systems, and mitigation. A "hands-on" exercise will make this the first gathering of its kind to provide lab-style instruction on interpretation of satellite and wind data, and on models of ash trajectory and dispersion. Representatives from the Boeing company, and a host of US government agencies and non-governmental organizations will attend. The workshop will include lectures, demonstrations, laboratory exercises, and a simulated-eruption exercise involving volcanologists, forecasters, controllers, dispatchers, and pilots.

A second international symposium on ash and aviation safety is being planned by the U.S. Geological Survey, organized by Marianne Guffanti. It will be held in Washington, D.C. in September 2003.

Large explosions occurred from MacKenney Crater on 16 January and 29 February 2000 (BGVN 25:01). Although an increase in SO2 degassing during February and March 2001 was reported (BGVN 26:04), the following provides additional information about activity from February 2000 through January 2002. The Washington VAAC reported that possible low-level ash around the summit was visible on GOES-8 satellite imagery on 30 May 2002.

Activity during March-November 2000. Hotspots on GOES images from the 29 February 2000 eruption were noted until 1 March 2000, after which they shrank until August 2000, although strong SO2 degassing continued at a level of ~1,500-2,000 metric tons/day (t/d).

In August 2000 ash eruptions took place for a week, after which a lava lake appeared in the summit crater during the last week of the month. The lava lake generated occasional booming sounds, ashfall, and the spattering of bombs. These were generally restricted to within MacKenney crater; they originated from an inner crater that was 20 m wide with 25-m-deep walls and ~50 m below the rim.

Gas emission increased to high levels beginning in November 2000, producing dense plumes that affected vegetation near the crater. The vigorous plumes were typically deflected by prevailing winds to the S and W where they damaged pine trees and corn fields in areas between El Caracol and Los Positos, 3 km W and 5.5 km S, respectively.

Activity during February 2001-June 2001. An increase in SO2 degassing during February and early March 2001 coincided with a significant increase in daily tremors (BGVN 26:04). On 22 February a local seismometer recorded >600 tremors/day, up from < 100 tremors/day in earlier weeks. At the same time, gaseous emission had increased from ~253 to ~550 t/d and there was visible magma in the crater. The SO2 flux was measured by COSPEC along the highway from Guatemala to Escuintla on 1, 15, and 28 February and on 8 March. The respective flux values in t/d were 951, 1,740, 1,448, and 1,673. These flux rates suggested that although lava outpouring was almost nonexistent, degassing of SO2 was quite high.

Nighttime incandescence from long-term lava lake activity was easily seen at a distance of several kilometers from August 2000 through June 2001 and was often visible from Guatemala and Escuintla cites (N and S of Pacaya, respectively). This marked a shift in eruptive style from the persistent Strombolian eruptions characteristic of Pacaya for several decades.

Activity during July 2001-November 2001. After June 2001, incandescence was no longer visible, but strong degassing continued, along with seismicity consisting of 250-380 B-type events/day. Erosion of the rim of the outer crater was accompanied by rock avalanches down the S flank of the cone. During September to October 2001 acid rain from the vigorous degassing affected areas to the N of Pacaya and burned leaves on vegetation in the El Cedro, San Francisco de Sales, and San José Calderas areas; residents in these areas reported that some birds had also died. On 31 October minor Strombolian explosions produced a little ash visible on satellite imagery during 1611-1820. A persistent plume reached 50-200 m above the crater, and ash fell S of the volcano. Since that time observers from INSIVUMEH reported only gas emission, and lava lake activity was not seen. On 14 November 2001 SO2 fluxes of 1,800 tons/day were measured at MacKenney Crater by INSIVUMEH.

Activity during January 2002. During field work on 12 January 2002, measurements were made of the active crater on MacKenney cone. Along a N-S transect the crater was 72 m wide at its narrowest point, and 94-100 m at its widest. The pit was ~80 m deep, and was the location of two sources of gas. Activity was characterized by persistent degassing. COSPEC measurements showed that SO2 emission rates remained high, ranging from 500 to more than 2,000 t/d. These rates were similar, but perhaps more variable, than one year earlier.

During 8-14 January 2002 geologists from INSIVUMEH and Michigan Technological University measured SO2 fluxes of ~1,200 tons/day in Pacaya's plumes (figure 33). Volcanologists who spent at night at the observatory reported seeing plume glow, presumably reflected light from a deeper lava source. The degassing produced a thick white plume, similar to that often generated by Masaya volcano in Nicaragua. The plume was seen drifting horizontally S from the crater on 29 January 2002 by volcanologists at the summit of Santa Ana volcano in El Salvador, 120 km E. This sustained level of degassing was in contrast to previous activity, during which the average gas emission rate for the 20-year period of 1973-93 was estimated by Andres and others (1993) to be a comparatively modest 260 t/d.

Figure 33. Aerial view taken on 14 January 2002 showing the upper part of Pacaya's Mackenney cone from the SE with a white gas plume blowing N. Courtesy of Matt Watson.

Although incandescence from the long-term lava lake ended after June 2001, SO2 emission rates remained high when measured in January 2002 (BGVN 27:07). On 30 May 2002, the Washington Volcanic Ash Advisory Center (VAAC) received a report from Guatemala City indicating that Pacaya was active. Satellite imagery showed possible low-level ash near the summit. A very thin SW-drifting plume was again visible in satellite imagery on 17 June 2002, but the composition of the plume was unknown. A faint hotspot at the summit was also visible on infrared imagery. Visual observation on the afternoon 24 August 2002 from the SW showed copious white steam emissions from the summit crater (figure 34).

The Washington VAAC reported that on 5 July 2003 at 0715, a very thin ash and/or gas plume was visible on satellite imagery at an altitude of ~ 3 km extending ~ 7.5 km SW. By 1430 the plume was no longer visible, possibly obscured by thunderstorm clouds in the area. INSIVUMEH reported that only steam was emitted. Visible imagery on 9 August 2003 showed a narrow plume below 3 km altitude extending SW from the volcano, but its composition was unknown.

Reports provided by INSIVUMEH during the latter half of October 2003 indicated that during 15-21 October constant steam and abundant emissions of water and gas were being blown to the NNW and W of the volcano. These emissions continued through the end of the month. On 23 October, during periods of visibility, observers saw a line of off-white smoke across the S flank, which was dispersed in the area of the lava field near the Chupadero and the Caracol rivers. The next day observers saw a heavy column of off-white smoke rising ~ 600 m over the MacKenney crater. The plume continued through 27 October, but only to a height of ~ 400 m. The heavy gaseous cloud continued at the same height through 30 October.

Frequent steam plumes through 2002 and 2003 indicated that Pacaya was active, although incandescence from the long-term lava lake ended after June 2001. During the latter half of October 2003 constant steam and abundant emissions of water and gas were being blown to the NNW and W of the volcano (BGVN 28:10). All of the following information is derived from the reports of Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH).

Throughout November and December 2003 and the first half of 2004, abundant clouds and columns of white and off-white gases and steam were expelled from Pacaya, generally reaching less than 400 m above the volcano and dispersing mostly to the W and SW; these were occasionally visible from Guatemala City, 30 km to the NNE.

During June, July, and August 2004, near-continuous tremor and frequent long-period earthquakes were recorded at seismograph station PCG (~ 1.4 km to the W of Pacaya). On 14 June, weak incandescence was observed in the central crater of MacKenney Cone for the first time since August 2000. Pacaya continued to expel off-white smoke and/or steam which usually drifted to the S and SW and rose to 150-300 m above the volcano. On 19 July, ejection of small lava fragments began to form a cone in the bottom of the central crater of MacKenney Cone.

During September-November 2004, tremor increased somewhat (from ~ 2mm in June, July, and September to 4-7 mm in December), and white steam and/or gas plumes rose 300-500 m above MacKenney Cone. Incandescence was observed throughout this time and lava clasts were expelled from the MacKenney Cone on 7-9 December.

On 3 January 2005, small expulsions of incandescent lava clasts rose from the central crater, and a narrow lava flow from the S rim of the crater reached 75-100 m down the flank. Station PCG continued to register tremor, and incandescence and white plumes persisted. On 10 January, lava flowed ~ 30 m from the SW rim of the central crater of MacKenney Cone. On 12 January, two lava flows, one to the S ( ~ 125 m) and one to the SW (~ 50 m) left the central crater. Observers saw incandescent lava fragments rising <10 m above the mouth of the intra-crater cone, and "smoke" whiffs rising from the MacKenney Cone. During the last 5 days of January 2005, numerous small lava flows descended the S and SW flanks of the volcano.

During February, March, and April 2005, incandescence, tremor, and minor lava flows continued. On 2 February observers reported that avalanches from the lava flow fronts during the previous days formed a debris fan covering about 2/3 of the SW flank. On 28 February expulsion of incandescent lava fragments reached heights of 10-50 m for brief periods. On 1 March INSIVUMEH recommended that park officials prevent tourists from climbing Pacaya because of avalanches, lava expulsion, and gas emissions. In March and April explosions of lava reached 100 m in height, and smoke/gas emissions continued.

Lava emission continued during May. On 4 May, three flows were active, extending up to 100 m down the SW flank and 150 m W in the direction of Cerro Chino. On 9 May two active flows from the base of the intracrater cone reached 200 m down the W flank. Plumes from the MacKenney Cone rose as high as 800 m above the crater. Ejection of incandescent material continued throughout the month. Lava flows moving to the SW and W in the direction of Cerro Chino reached lengths of 150-250 m.

During early June, incandescent lava clasts were ejected as high as ~ 75 m above Pacaya's crater. An intra-crater lava flow extended ~ 300 m from the SW base of the central cone. On 6 June, a lava flow traveled ~ 200 m down the volcano's W flank. By 27 June a lava flow extended ~ 300 m down the SW flank. A white steam column rose ~ 150 m over the central crater and drifted SW. Incandescent lava expulsions reached heights of 15-50 m. On the night of 27 June two rivers of lava, 75 and 150 m long, were observed in front of Cerro Chino. Constant expulsions of pyroclastic material rose 20-30 m above the crater.

Lava flows in July traveled 200-300 m down the SW flank. Small plumes emitted from the volcano's central crater rose to low altitudes. Avalanches of incandescent volcanic blocks produced small ash clouds to low levels.

During 7-11 September, occasional Strombolian activity occurred. Volcanic bombs from two craters rose up to 30 m above their rims. Incandescence from lava flows on the SE flank was visible on several nights.

During 2005 lava overtopped the collapse scarp to the inhabited N slopes

Our last Bulletin report discussed events at Pacaya as late as September 2005 (BGVN 30:10). Starting in 2005, lava flows from the active cone (MacKenney cone) substantially altered the local morphology and the consequent risks. The larger Pacaya complex's SW side is marked by an arcuate collapse scarp with relief up to 200 m. In 2005, for the first time, lavas accumulated in sufficient thickness to cross the NE portion of this barrier. If lavas advance substantially N from this point, they would descend steep slopes and could endanger hikers and residents.

Gustavo Chigna, of the Instituto Nacional de Sismologia, Vulcanologia, Meteorología, e Hidrologia (INSIVUMEH), mapped the substantial lava field N of the summit in 2008. In places, the flows that accumulated during 2005-08 reached 100-150 m thick (figure 35). The flows chiefly emerged from a new fissure on the upper NNE flank, constructing a protrusion from the MacKenney cone. As the lavas advanced they curved W, many ultimately reaching the N to NW sides of the active cone. Material venting within that crater sometimes formed small ephemeral cones that reached above the high point on the enclosing crater rim, but they always collapsed later.

Figure 35. A topographic map of the Pacaya area from 1970 annotated to show lava flows emitted during 2005 through mid-2008. From a new fissure on the upper NNE flank, flows curved W, many reaching the N to NW sides of the active MacKenney cone (labeled "Pacaya"). Lava accumulated in the depression between the MacKenney cone to the S, the Cerro Chiquito and Cerro Grande cones to the N and NE, and the Cerro Chino cone to the NW. Between the latter three cones lies a comparatively flat area informally called la meseta; lava flows had crossed substantial portions of this area by 2006. Eruptions also deposited some lava on the cone's E side and in MacKenney crater. The upper margin of the collapse scarp (CS) is also indicated (light line). Ruled squares are 1 km to a side; heavy contours are 100 m. Courtesy of Gustavo Chigna.

About 1,100 years ago Pacaya's SW side underwent a sector collapse, an event where a major part of the edifice collapsed, forming a debris avalanche that reached the Pacific coastal plain (Siebert and others, 2006). The edifice still bears an enormous scarp from this event. Within the horseshoe curve of this scarp, the MacKenney cone subsequently grew. It eventually rose to sufficient height to form the summit of the multi-peaked complex.

Although thick, rough-surfaced lava has emerged for years from the MacKenney cone to flow in various directions downslope, those during 2005-06 advanced in a new and unexpected way. In a manner similar to previous episodes, some of the N-flowing lavas descended into the depression and were confined to curve around the moat. In contrast, other lavas cooled and accumulated sufficiently to fill this portion of depression. The lavas ultimately overtopped the collapse scarp, and flowed onto the ancestral cone (figures 35 and 36).

Figure 36. (top) A view in 2005 from Pacaya's summit (on the MacKenney cone) looking to the N with Cerro Grande the largest peak in view. Heavy tephra fall deposits covered the landscape in the field of view, the result of many years of Strombolian activity. At this stage the collapse scarp (steep ridge across the bottom third of the photo) still formed a significant topographic boundary. Activity during the photographer's visit was only fumarolic. (bottom) Night image of the same scene in December 2007; here the collapse scarp is absent, owing to inundation of the area by viscous rubbly lava. The narrow fingers of lava at distance reside on the meseta. Photos courtesy of Richard Roscoe (www.photovolcanic.com).

Since restarting after about 76 years in 1961, the volcano has erupted lavas with only occasional breaks of months to a few years. The latest eruptive pulse began in 2004. The summit elevation of the MacKenney cone has varied due to the cone's repetitive growth and construction.

MODIS thermal alerts from the MODVOLC website were issued frequently for Pacaya during the reporting interval. The only months without alerts took place during the six-month interval of September 2005-February 2006, and December 2006. More precisely, these gaps in alerts spanned 29 August 2005-10 March 2006 and 29 November 2006-23 January 2007 (all local dates).

Pacaya resides just outside the southern topographic rim of Amatitlán caldera and ~ 30 km S of central Guatemala City (Lima and others, 2000). Maps of the setting and volcano appeared here most recently in BGVN 24:02 and 25:01. The National Park that includes Pacaya was created in July 1963 and it is a popular tourist destination (Bohnenberger, 1967). The trail along and to the meseta was crossed by lava flows during 2006 and later, hampering access and leading to risk concerns (figures 37-39).

Figure 37. (top) Steaming MacKenney cone at Pacaya as viewed looking S from the main trail in 2005 from an area just below and S of the meseta. Note bending tree and structure(s) along depression in foreground. (bottom) Very similar view taken in December 2007. A large lava cone had formed on the N flank of MacKenney and lava flows had reached the trail during 2006 and 2007. The beleaguered tree still stands. Photos courtesy of Richard Roscoe (www.photovolcanic.com).

Figure 38. Nighttime (time-lapse) views of Pacaya's MacKenney cone as seen looking W in December 2007. (top) Summit incandescence and lava flows emerging from the cone's N flank. The latter constructed a lava cone that supported additional lava flows. (bottom) Flat-topped, antenna-laden Cerro Chino of the Pacaya complex is at lower right, and at distance in background from right to left reside Agua, Acatenango, and Fuego stratovolcanoes. Photos courtesy of Richard Roscoe (www.photovolcanic.com).

Figure 39. Daytime view of Pacaya's descending lava flows heading N on the shield area, 4 June 2006. The ribbon of lava trends remained linear, despite the flow field's surface irregularity. The margins appear partly contained by levees. Numerous other zones of glowing lava reside in the distance at lower elevation. Photo by AnaLu de MacVean.

INSIVUMEH reports. Gustavo Chigna (INSIVUMEH) sent a report summarizing activity during 2005 through May 2008. He noted Strombolian activity during 1961-2000, typically with two to three paroxysmal eruptions each year. Those eruptions included falls of both ash and ballistic blocks, production of lava flows, and abundant gases escaping at the vent in the MacKenney cone's central crater. Pyroclastic flows were also mentioned, but without details. This eruptive pattern changed in the year 2000. The paroxysmal eruptions of January 2000, and 29 February 2000, and those continuing until September 2008 all chiefly consisted of steam-rich and ash-poor explosions.

During January-March 2005 a new phase of activity developed where the active cone emitted small batches of lava. This phase accompanied the repeated building and destruction of intracrater cones.

Observers in March-April 2005 saw the growth of N-S oriented cracks on the MacKenney crater, reaching 100-150 m in length and sometimes longer. Many of the cracks were 30-70 cm wide at the surface, and inspection revealed their open portions penetrated downwards about 1-8 m. Associated with these cracks, a depression became established on MacKenney cone's N side.

A new vent began emissions during a few days in mid-March and on 1 April 2005. Lava emerged from cracks on the cone's ENE side. In just a few days, the flow field from this vent grew to ~ 800 m long (figure 35). It curved to the W following the moat or valley floor (a comparatively flat area also called los llanos). By about 1 August 2005 this venting had sent many lava flows into the adjacent parts of the depression on the MacKenney cone's N flank. The rapid rate of lava accumulation during August filled up much of this part of the depression and eventually overtopped the scarp.

As the flows began to advance over the collapse scarp, alarm spread among residents of San Francisco de Sales, the town 1 km N of the flow front. The flows soon returned to advancing more to the W in the area confined by the collapse scarp and in the depression along los llanos.

The following year, after the 29 August 2005 and 10 March 2006 interval without thermal alerts, lava advanced onto a higher part of the meseta adjacent to a monument. This event is documented in two photos taken 27 July and 3 August (figure 40). Photos taken in August 2006 of the meseta show that the trail largely flow-covered (figure 41).

Figure 40. Cooled lava flows from Pacaya as seen looking along the collapse scarp to the S on (top) 27 July 2006 and (bottom) 3 August 2006. The MacKenney cone is out of the picture to the right. The flows on 27 July had nearly completely filled the depression N and NE of the meseta. By 3 August a flow crossed onto the meseta. Courtesy of Gustavo Chigna.

Figure 41. Two photos of Pacaya looking SE showing lava flows advancing across the meseta. (top) A flow lobe lay across the main trail on 8 August 2006. (bottom) Visitors confronting a new scene on 11 August 2006 at the meseta where the former trail was largely covered by rough-surfaced lava flows. Courtesy of Gustavo Chigna.

The lava amassed between the MacKenney cone and meseta represented a rapid and remarkable morphologic change. Meseta historically provided an elevated viewpoint from which observations of Pacaya could be made. As a result of the new morphology, and assuming similar ongoing eruptions, hazards now confront N-flank villages and the main trail access route. INSIVUMEH plans to review hazard maps and strategies for this area.

Our last report on Pacaya was in August 2008 (BGVN 33:08), which covered activity through September 2008. Unless otherwise indicated, the following report is a compilation of reports from Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH).

During 8-14 October 2008, the pattern of previous activity continued with multiple lava flows on the W and SW flanks of MacKenney cone that traveled a maximum distance of 250 m and continued to fill in the area between the cone and Cerro Chino crater to the N. Avalanches occurred from the lava-flow fronts on 8 October. Fumarolic plumes drifted SW.

Based on analysis of satellite imagery, the Washington VAAC reported that on 2 November 2008 a possible ash-and-gas plume was emitted from Pacaya and drifted E. On 3 November, INSIVUMEH reported that fumarolic plumes drifted S at a low altitude. Ash occasionally entrained by strong winds drifted S. Multiple lava flows on the S and SW flanks of MacKenney cone traveled a maximum distance of 400 m on 3 and 4 November, and continued to fill in the area between the cone and Cerro Chino crater to the N. Fumarolic plumes drifted E on 4 November. On 20 November fumarolic plumes from Pacaya's MacKenney cone drifted S at a low altitude. Ash occasionally entrained by strong winds drifted S. Multiple lava flows on the S, W, and SW flanks of the cone traveled 50-300 m during 20-21 and 25 November.

On 12 December 2008 fumarolic plumes from Pacaya's MacKenney cone drifted NE at a low altitude. Three lava flows, 150, 250, and 800 m long, were observed from the S. Seismic data indicated small explosions at the crater.

On 30 January and 3 February 2009, white and blue fumarolic plumes from MacKenney cone drifted S and SW at a low altitude. One lava flow, 75-100 m long, traveled down the SW flank.

On 12, 16, and 17 March 2009, fumarolic plumes from MacKenney cone drifted S at a low altitude. Lava flows, 25-200 m long, traveled S, SW, and W. Explosions during March ejected greater amounts of material that was deposited in the crater, enlarging the cones there. On 23 March, visual and audible changes in Strombolian activity were noted. Vigorous degassing produced sounds resembled airplane engines.

In a report issued on 3 April 2009, INSIVUMEH stated that Strombolian explosions from MacKenney cone during the previous few days ejected material 25 m into the air. On 2 April, lava flow volume increased, sending four lava flows W and one SW; the flows traveled 25-200 m. The seismic network detected tremor and explosions. On 6 April, lava flows on the W flank traveled 150-300 m, causing lava to collect on the SW flank. Activity from MacKenney cone was continuous; one cone emitted gas and explosions about every 5-10 minutes, and a second cone ejected tephra 25 m high. On 7 April, one lava flow traveled 150 m W and one traveled 200 m SW. INSIVUMEH recommended that CONRED coordinate with authorities in Pacaya National Park to restrict visitors from climbing Pacaya. On 24 and 28 April, INSIVUMEH reported gas emissions from Pacaya's MacKenney cone; occasional ash explosions ejected tephra 15-25 m high. The seismic network detected tremor and explosions. A small spatter cone being built in the S part of the crater was 4 m high. Rumbling noises were heard 3-5 km away and degassing produced sounds resembling airplane engines. Lava flows traveled 50-400 m down the SW flank and fumarolic plumes drifted S. This pattern of activity continued throughout May 2009.

For the remainder of 2009, the pattern remained much the same. On 5, 8, and 9 June 2009, white and blue fumarolic plumes from Pacaya's MacKenney cone rose to as high as 400 m and drifted, S, W and SW. Multiple lava flows up to 600 m long, were emitted from an area on the lower S flank, SW from the main edifice and traveled S, SW and W. Incandescence at night was noted on 20 November and 18 December.

Similar activity continued in 2010. On 8, 11, and 12 January 2010, white and blue fumarolic plumes from Pacaya's MacKenney cone rose up 400 m and drifted S and SW. Multiple lava flows on the S, SW, and W flanks traveled 25-200 m. Incandescence was noted at night from one of the inter-crater cones on 8 January and from MacKenney cone on 11 and 12 January.

Introduction. Pacaya, which in recent years has consistently erupted olivine-bearing high alumina basaltic lavas, erupted with remarkable violence on both 27 and 28 May 2010 with an explosion on the 27th lasting ~45 minutes. This was followed by a smaller explosion the next day that generated a plume assessed from satellite and meteorological data as reaching 13 km altitude. In this report we describe those events as explosions in order to distinguish them from the ongoing, decades-long, and often effusive eruption generally seen at Pacaya. The terms ‘explosion’ and ‘explosive’ appear warranted given such factors as the suddenness of escalation, the ~13 km plume altitude (~10 km over the summit when measured during the weaker explosion on the 28th, the density of projectiles, and the scale of the tephra fall. The term explosion seems consistent with common practice (Sparks, 1986; Fiske and others, 2009).

The following report emphasizes Pacaya’s behavior in 2010, including the 27 and 28 May explosions and impacts continuing into early June 2010. Our last report (BGVN 34:12) discussed behavior into mid-January 2010. Some of the reporting came from reports of Guatemalan agencies (eg. INSIVUMEH and CONRED, acronyms spelled out in the Information contacts section at bottom), newspapers (eg. Prense Libra, 2010a, b), videos and photos, and cited manuscripts and papers. It especially benefited from a draft manuscript prepared by Rüdiger Escobar Wolf (REW, 2014) and graciously provided to Bulletin editors. REW also provided reviews, insights, and numerous tailored graphics but bears no responsibility for possible errors induced by Bulletin editors.

The explosions were preceded months to weeks earlier by extra-crater venting of lava flows on the E and SE flanks. The lava flows covered substantial areas after emerging effusively at two widely spaced vents in atypical extra-crater or crater-margin locations.

Following the Introduction, this report’s subsections address the following topics: (1) the Guatemalan hazard agency CONRED’s reports, (2) a sample of available video and photo documentation of Pacaya’s behavior, (3) events prior to the 27 May explosion, (4) the explosions and some of the impacts, (5) the seismic record showing the pattern of escalation around the time of the explosions, (6) a brief summary of the critical initial aviation reports, and (7) a geotechnical slope stability study that suggests gravitational instability at Pacaya, particularly owing to the cone’s magma pressure and seismic loading.

Pacaya , which has a record of eruptions dating back over 1,600 years, has been erupting the majority of the time since 1961, often emitting rough-surfaced lavas but also occasionally discharging explosions. The centerpiece of the National Park of the same name, it is the most often climbed volcano in Guatemala. There have been 69 prior Smithsonian-published reports describing behavior from 1969 to early January 2010 (CSLP 03-70 to BGVN 34:12). REW (2013) ranked the 27 May explosions as sub-plinean and the associated lava emissions as the largest since similar events in 1961.

Figure 42. (Top) A map showing Pacaya’s location in Central America. (Bottom) A map emphasizing Pacaya’s location with respect to the central portion of Guatemala City (red square labeled ‘Guatemala’). The larger combined urban area associated with that Capital city stretches well beyond the square symbol and contains ~3.5 million residents. AmatitlÁn was heavily damaged by Pacaya’s May 2010 ashfall and the knock-on effects of Tropical Storm Agnes that arrived two days later. Top map taken from Morgan and others (2012); bottom map revised from a base map found online at Ezilon Maps.

The larger tephra blanket spread N, covering an area of more than 1,000 km2 including the bulk of the Guatemala City metropolitan area, the largest city in Central America, population ~3.5 million. The City’s center lies ~25 km NNE of Pacaya’s summit but a 5-km-wide strip of urban and suburban development now stretches from its older core (red square, figure 42)to ~9 km N of the summit. The tephra shut down La Aurora, the county’s primary international airport and among the region’s busiest, for 5 consecutive days.

The 27 May 2010 explosion destroyed or damaged nearly 800 houses in nearby communities, forcing ~2,000 residents to evacuate and injuring 59 people. A high density of ballistics fell on nearby hamlets and villages, particularly those 2.5-3.5 km N of the MacKenny cone (El Cedro, San Francisco de Sales, and Calderas). The ballistics had sufficient mass and velocity to puncture roofs with a density on the order of one puncture per square meter in some places. Many more smaller ballistics bent but did not penetrate the corrugated sheet metal roofs common in many of the region's dwellings. Some of the ballistics were sufficiently hot to start fires.

Ash caused widespread damage locally, and up to ~8 cm of ash fell on parts of metropolitan Guatemala City, the nation’s capital, centered ~35 km NNW of Pacaya. Up to 20 cm of tephra accumulated at and near Pacaya. According to available census data, the population within 10 km of Pacaya was 57,000 (John Ewert, USGS-CVO, personal communication).

Accounts from Guatemalan meteorological stations reported that detectable ash from the 2010 explosions fell as far away as the Caribbean coast. Brianna Hetland was both a graduate student in volcanology and a US Peace Corps Volunteer in Guatemala during 2010-2012. Hetland noted in a message that she had spoken with another Volunteer who said ash had blanketed his neighborhood near Coban (in Samac, Alta Verapaz) ~180 m N of Pacaya (figure 42, bottom). Hetland documented post-eruptive conditions at Pacaya, composed a blog on the impact and clean up, and gave a talk on those aspects as well as multifaceted monitoring conducted by fellow students and faculty at Michigan Technological University (Hetland, 2012a, b; Walikainen, 2010).

Some of the impacts of the freshly fallen ash were amplified and other impacts were diminished by heavy rains and flooding due to Tropical Storm Agatha that struck the region 2 days later, with some areas receiving 0.9 m of rain. The floodwater run carried ash that dislodged debris, clogged drainage systems, left thick deposits on valley floors, and damaged many bridges. The scale of the combined disasters led to more analysis of hardships, mitigation, and economic impact than usual at many eruptions, as exemplified by the detailed assessments by Wardman and others (2012). Those authors visited in the aftermath from New Zealand in order to study impacts that might be analogous to hazards elsewhere. They found that one moderating impact of the rain was to cee crops, which were washed clean of ash and residual acids. The authors also found that that a prompt and efficient cleanup was initiated by the Capital municipality to remove tephra from the 2,100 km of roads in the Capital. An estimated 11,350,000 m3 of tephra was removed from the city’s roads and rooftops.

Diminishing strombolian activity and lava flows in the crater area continued into at least late June 2010. By this time the emissions had become more like the generally effusive decades-long eruption, which was still ongoing when this was written in late 2014. In addition to the information here, Pacaya’s discharge rates have been summarized for the years 2004-2010 on the basis of infrared satellite images (Morgan and others, 2013). As would be expected, a strong peak in radiance developed in late May 2010.

REW (2013) noted one death attributed to the explosion and tephra fall and 179 deaths attributed to the Tropical Storm. Two people died at Pacaya days prior to the explosion of 27 May 2010. Wardman and others (2013) mentioned two further deaths due to people cleaning tephra from roofs.

Geochemical analysis of material erupted on 27 and 28 May is not yet reported. As background, Matías and others (2012) describe Pacaya’s recent lavas as all high-alumina basalts with SiO2 contents of 50-52.5 weight percent and MgO contents of 3-5 weight percent. Common phynocrysts (visible minerals) included plagioclase, olivine, and opaque minerals (Conway, 1995). There is a slight variation of CaO in this group of lavas, which suggests a phenocryst enrichment or depletion. The lava compositions have remained broadly similar since 1961, and for many previous lavas as well, although some more felsic compositions are represented at older flank eruptions (Eggers, 1971).

CONRED reports. Perspective on the disaster can be gained from the chronology and content of announcements issued by CONRED (the Guatemalan agency for disaster reduction; Coordinadora Nacional para la Reducción de Desastres, table 4). These will be referred to in text by “CONRED” followed by their bulletin number.

Table 4. A summary of key CONRED information bulletins issued relevant to Pacaya’s May 2010 eruption (http://conred.gob.gt/). After Escobar Wolf (2012) in addition to a similar table by Wardman and others (2012). Not all bulletins are included in this table.

Began to mobilize staff to villages near volcano around 1500 on the 27th, to discuss and implement pre-emptive evacuation. Seven shelters were prepared in San Vicente Pacaya to accommodate refugees.

When the paroxysmal phase of eruption started (after 1900), evacuation of villages to the W (El Rodeo and El Patrocinio) was already underway, however, tephra and ballistics were dispersed primarily to the N and the villages of El Cedro, San Francisco de Sales and Calderas were the most severely affected.

28 May 2010

731

Declared Red Alert. As of 1239 on the 28th over 1600 people had been evacuated from the villages of San Francisco de Sales, El Rodeo, El Patrocinio, El Cedro, Calderas, and Caracolito. They moved to San Vicente Pacaya.

Civil Aviation authorities closed La Aurora International Airport due to tephra fall. The Ministry of Education closed schools in Escuintla, Sacatepequez and Guatemala departments. Access to the National Park remained restricted.

The municipality-level response agency (with a similar name, COMRED, not CONRED) was activated in Villa Canales. It set up shelters in the municipal auditorium, a church, and the municipal hall.

In the afternoon at 1424 on the 28th, high eruptive vigor resumed and tephra again fell on Guatemala City, but in much smaller quantities than during the previous day.

29 May 2010

748

By this time, a total of 2635 people were in shelters due to the eruption; ~400 houses had been slightly damaged and 375, severely damaged.

27 May 2011

1673

One year later; a retrospective summary of civil defense responses to the eruption and the larger engulfing disaster, tropical storm Agatha.

Events prior to the energetic 27 May explosion. Figure 43 highlights Pacaya’s vent locations (1961 to 2009 vents as green dots), including the two new E and SE flank vents that emitted lava flows (red areas).

Figure 43. Simplified geological map of Pacaya, based on cited references, INSIVUMEH mapping, and GOES satellite data. The key at right calls attention to features such as the collapse scarp forming the N and E of margin of the main crater and the lava flows of prehistoric age (Eggers, 1969, 1972; Bonis, 1993) through about mid-2010. Migrating vents mapped during 1961-2012 (Matías, 2010; Rose and others, 2012) appear as dark-green dots (many clustered on or near the MacKenney cone’s summit). The red areas on the SE flank and E flank represent lava with the noted age constraints from REW’s analysis of satellite data. The SE flank vent had emitted by mid-2010 a field of lava approaching the size of the 1961 Cachiajinas lava flow (purple). The latter flow both vented and advanced within Pacaya’s collapse scarp. In contrast, the SE flank flow was the first in historical times to vent and flow outboard of the scarp. The cone residing on Pacaya’s NW rim, Cerro Chino, enters discussion frequently in this report. Note the depression (notch or trough) here labeled “New fissure like structure.” Map created and provided by REW.

Changes in eruption behavior preceded the 27-28 May explosions by several months.

From 2004 to around the end of 2009, Pacaya’s eruptive intensity was often low. A clear sign of changes took place in February 2010 when lava flows emerged at vents on the S and SE flanks (table 1). These vents sit well outboard of the usual points of lava emission, which have in recent decades been limited to spots within the central crater, an area bounded by a large engulfing collapse scarp (a Somma rim; Eggers, 1969; figure 43). The two previously mentioned deaths occurred on 18 April when, according to the news, they were hit by a rock avalanched caused by an explosion. By 17 May, SE flank lava flows had reached 1.5 km long and the Park began restricting access (table 1).

The scene on the SE flank appears in figure 44.

Figure 44. Pacaya’s SE flank eruption as seen during the day on 27 May 2010. The ultimate distribution of lavas appears on the preliminary map by REW (2014). Image courtesy of Gustavo Chigna (INSIVUMEH).

Earlier on the 27th (prior to the explosion), INSIVUMEH volcanologist Gustavo Chigna looked out over the crater area and counted at least 16 distinct vents emitting lava. Chigna was surprised, and his’s comment was something like, ‘It looked like water gushing out of a sieve.’ That scale of new extrusive sites helped alert authorities that the volcano’s behavior had escalated well beyond the norm and led to restricting public access to Pacaya.

During the 5 years prior to the 27 May 2010 explosion, sporadic vent openings limited to the MacKenney cone and adjacent areas (particularly the N crater) extruded lava flows (green dots, figure 43). Many of the resulting lava flows were each only active for periods of days to months. INSIVUMEH sometimes reported multiple simultaneous lava flows from distinct vents on the cone, which occurred, for example, during April 2009. Most of the lava was confined to the main crater or portions downslope and W of the E-bounding collapse scarp. The case in 2005 illustrated that the topographic boundary associated with the NE segment of the collapse scarp had diminished in places to the point where lava flows could cross the scarp (BGVN 33:08).

Around January 2010, Gustavo Chigna (INSIVUMEH) indicted the end of mainly lower effusive activity ongoing since 2004. The new upsurge fed several lava flows from vents on Pacaya’s main cone. In harmony with this comment, the video by Crossman (2009) indicates that on 24 December 2009 the volcano emitted considerable lava. Venting was effusive and at both the MacKenney cone’s summit and base. Visible plumes were nearly absent.

Table 5 lists a small sample of available videos taken at Pacaya that aid in documenting its behavior. The table includes videos taken before, during, or shortly after the 27 May explosion, with the two pre-explosion videos capturing behavior relevant to this subsection. The videos from other parts of the table are discussed in appropriate sections below.

Table 5. Some photos and videos that advance understanding of Pacaya behavior during December 2009 to about 2 June 2010 (a week after the explosion). The cases presented are a sample, not an exhaustive list. Compiled by Bulletin editors.

Video (V) or Photo (P) and source

Date acquired / Date posted if clearly stated)

Title; Content; URL

How cited in text of this report

V; Patrick R. Crossman

24 Dec 2009 / 24 Dec 2011

Title: “Hiking the Pacaya volcano in Guatemala”

This video chronicles a group visiting Pacaya amid ongoing effusive volcanism in comparatively calm conditions and with people in many scenes. Some parts of the video depict a narrow (1- to 2-m wide), channelized, slowly moving lava flow. That flow appears to vent near the base of the MacKenney cone, devoid of visible plume, and traverses a region of low incline. The path of the molten flow is sinuous rather than linear. The visitors roast marshmallows in radiant heat from the flows. The video also cuts to scenes at the MacKenney cone’s summit, where a larger flow several meters wide vents in a stable, effusive manner, also devoid of an associated plume.

[Date confirmed with Moon and by comparison to his dated still photos]

Title: “Pacaya Volcano, Guatemala [1080p HD]”

Close up views showing copious lava flowing down the E flank from the new vent there. Accompanies GPS record of hiking track and still photos. Music accompanies the video. Dovetails with a Landsat image from about a week earlier, which also documents the E flank lavas. See text for more discussion.

Title: “Raw video of damage caused by volcano eruptions in Guatemala and Ecuador”;

The video shows, for Pacaya, images of advancing lava flows and some distant views of the volcano in daylight with a moderate plume above it. There are many scenes of damage, evacuation, and human impact, including ash-loaded corrugated metal roofs that buckled; ash on airliners; brigades of people sweeping and carting off ash from city streets and an airport runway; and children sheltering in a relief center.

“In just the past seven days, residents of Guatemala and parts of neighboring Honduras and El Salvador have had to cope with a volcanic eruption and ash fall, a powerful tropical storm, the resulting floods and landslides, and a frightening sinkhole in Guatemala City that swallowed up a small building and an intersection. Pacaya volcano started erupting lava and rocks on May 27th, blanketing Guatemala City with ash, closing the airport, and killing one television reporter who was near the eruption. Two days later, as Guatemalans worked to clear the ash, Tropical Storm Agatha made landfall bringing heavy rains that washed away bridges, filled some villages with mud, and somehow triggered the giant sinkhole--the exact cause is still being studied. (34 photos total).”

Helicopter views of flight generally towards, and then at, Pacaya, which was still in eruption, with initial views showing Agua volcano and parts of Lake Amatitlán. Low weather clouds covered extensive areas. This video captured a decidedly non-vertical, denser black plume from Pacaya feeding a lighter, tan colored more massive plume that appears to drop ash as it is carried to ten’s of kilometers downwind (directed E-SE-S). Shots include those of Cerro Chino and antenna towers there, and widespread steaming on the MacKenney cone that coalesced into large steam clouds low over much of the central crater area.

(Narration by news reporter referring to explosion as 1 week ago, thus the ‘About 2 -3 June’ date in the previous column.) According to REW , this video shows lavas emitted at the new SE flank vent. Remarkable images, some seemingly shot from helicopter and others from the ground, showing copious channelized lava flows moving rapidly downslope to the SE. At the vent area there are three small vents discharging spatter from coalescing cones with very steep sides. Their glowing summit craters gave off occasional eruptions as well as occasional puffs of gases, glowing spatter, and possibly flames. Some shots show incandescent lava flows several kilometers long. Rising plumes sometimes display toroidal motion, rotational behavior reminiscent of dust devils.

Figure 45 shows one of several Landsat views of the E flank in an infrared image acquired on 23 March 2010. It showed high thermal radiance in a narrow linear thermal anomaly headed E outboard of the usual eruptions confined to the crater. The E-flank area is devoid of vegetation, which rules out a local fire there, meaning that the anomaly was due to a lava flow. The number of clear (cloud-free) views of Pacaya available during March through June was limited. REW plotted this anomaly in a KMZ file format (red line, figure 46).

Figure 45. A Landsat 7 thermal image of Pacaya on 23 March 2010 showing high heat flux as red. The small red area is on the MacKenney cone. The larger red area is a lava flow that had extended E. A site visit and video by Moon (2010) on 1 April (8 days later) confirmed lava flows on the order of 2-4 m wide. Black and marginal gray areas are older lava flows; green areas are vegetated with some cultivated or pasture land in shades of brown. This image contains artifacts in the form of gray diagonal stripes. The stripes are due to the failure of the Scan Line Corrector (SLC), which compensates for the satellite’s forward motion. Courtesy of REW.

Figure 46. A Google Earth view of the land surface looking radially outward (E) down Pacaya’s E flank (N is to the left). The red line indicates the location of the lava flow axis from heat flux in Landsat images. The flow’s source was at or very near the collapse scarp. The yellow line indicates the film crew’s 1 April 2010 excursion route recorded with GPS as they approached the lava flow, filmed it at close range, and then headed back towards the trailhead (Moon, 2010). For scale, the lava flow is ~0.3 km long. Graphic files, analysis, and compilation created and provided by REW.

The new E flank (extra-crater) lava flow documented by Landsat on 23 March was the subject of a video by Moon (2010) taken on 1 April (table 1; see their excursion route on figure 46). The footage was shot during daylight hours at high resolution [1080p HD] and later processed to obtain vibrant red, orange, and yellow colors.

The discharges were effusive and few visible emission clouds accompanied the lava flows seen in the video. A dark plume remained above the MacKenney cone’s summit.

As seen in fs 47-50, the lava documented by Moon (2010) in photo and video was several meters wide and passing over irregular terrain. As seen from a distance (eg. figure 47), some sectors of the flow’s channel stood well above the surrounding landscape. In the area visited, the lava remained confined behind jumbled but effective levees as it passed through and over the a’a (rough textured) flow field.

Figure 47. A 1 April 2010 photo of Pacaya’s E flank lava flow seen in the distance as it descends across an a’a flow field. Courtesy of H. Paul Moon (see table 5).

Figure 50. A still closer view of Pacaya’s E-flank lava (taken from just a few meters away), which was moving swiftly. In his YouTube notes on his teams 1 April 2010 visit Moon commented that “the heat was so intense that I could only hold out for brief shots, needing to turn away regularly to avoid getting scorched.” Courtesy of H. Paul Moon (see table 5).

Figure 51. An oblique Google Earth view of Pacaya looking roughly WNW. At left in orange appear the upslope areas of the fissures that fed the SE flank lava flows. Farther NE (to the right) appear another set of fresh black lavas that reside on the upper E flank. The green line traces high heat emissions REW found in Landsat imagery from 23 March 2010, the same lava flow that had been the subject of Moon’s video ~8 days later. Both sets of flows and vents were the first clearly documented to extend E of the collapse scarp in historical times. Analysis, compilation, and topographic files all provided by REW.

On 18 April 2010, according to a news report in the newspaper Prensa Libre, a Venezuelan tourist and her Guatemalan guide died on Pacaya. The news report stated the deceased were in the area of high risk when struck by material released from an explosion. Some of the other 14 people on the scene sustained injuries.

On 17 May 2010, observers saw abundant lava escaping from a new SE-flank vent (CONRED 708). A mound had formed at the vent area. The lava from this vent had by 17 May extended as far as 1.5 km. As seen on figure 43, the SE flank lava flows and their fissures ultimately fed lava flows trending roughly S for ~2.5 km then turning sharply (~90 degrees) to the W and extending in that direction another ~2.5 km.

CONRED 708 made a recommendation to the Pacaya National Park authority to restrict visitor access to the lava flows. The 17 May report noted that Pacaya’s activity was considered to be relatively high, but it left out language suggesting a crisis at this point. According to the press, access to the volcano was restricted following the recommendation.

On 17 May, the newspaper Prensa Libre featured an undated night photo of the MacKenney cone taken from the N, presumably of this stage of Pacaya’s eruption. It showed a dense spray of glowing material thrown from the MacKenney cone’s summit and rising hundreds of meters. The cone’s N rim contained a recently formed V-shaped notch (or trough). Out of that notch poured a broad lava flow. Several hundred meters down the MacKenney cone’s N face, the broad flow split into two flows descending the cone’s steep face on diverging paths. The notch in the cone stands out as a clear morphologic change associated with this time interval (~10 days prior to the 27 May explosion), and as will be seen below, it served as a conspicuous vent site for the fissure emissions documented during the explosions.

The day before the explosion, on 26 May, eruptive and seismic intensity both increased markedly. An eruptive plume reached 1 km above the vent and fine tephra fell on villages around the volcano (CONRED 726 on 26 May, table 1). CONRED recommended fully closing Pacaya National Park, and they warned aviation authorities of airborne ash near Pacaya. No call was yet made to evacuate residents living adjacent Pacaya.

Vigorous explosions starting 27 May 2010. Pacaya’s eruptive vigor increased to the point of strong strombolian eruption, with the initial increase noted on the 27th in a morning report in Prensa Libre. More intense explosions occurred at around 1500 when observers noted explosions discharging about once per second and saw glowing material thrown ~1.5 km above the crater, and taller rising dark clouds carrying finer tephra that dispersed over nearby villages.

The exact start time of the intense 27 May explosion is variously reported, but available visual observations suggested to REW (2014) that it was during the interval 1800-1900. CONRED 729 indicated the climax (the explosion)began at 1900. Seismic data, discussed in a subsection below underwent the highest (RSAM) amplitudes during 1730-1830 local time on the 27th. Aviation reporting of satellite data on eruptive plumes, discussed in a subsection below, was initially ineffectual for the 27th owing to above-lying weather clouds.

What is clear is that the explosion late in the day on the 27th drove forth intense fire fountaining and vigorous ejection of tephra and ballistics.

Figure 52 shows a broad fire fountain frame taken from a Youtube video posted on 28 May—but it lacked an acquisition date (RT news channel, 2010a). REW interprets this video as taken during the major climax (explosion) during the night on the 27th. The eruption was clearly of fissure style at this point but the upper extent of the glowing material was possibly masked by ash clouds. Some of the textures within the glowing region are explained in the f caption and in the text below.

Figure 52. A frame captured from a news video taken at night from Pacaya’s NNW side documenting powerful curtain-style emissions (fire fountains) from the main crater area (which includes the MacKenney cone). The foreground consists of the dark silhouette of Cerro Chino (indicated on figure 43). Some of the tall antenna towers there appear as narrow vertical dark streaks backlit by the brighter orange fire fountains. Many of the towers and radio shacks on the ground near their bases were destroyed. Taken from RT news (see table 5 (RT News, 2010 (a)).

REW described the video source for figure 52 as taken looking at Cerro Chino (indicated on figure 43) from at or near the town of El Cedro, ~3 km to the NNW of the vent. The diffuse zones of near darkness in the midst of the fountains are rising ash clouds locally diminishing the glow. Thus it is clear from the dynamics seen on the video, that the glow of higher reaching clasts in the upper portions of this image could possibly be masked by dense ash plumes.

On the video, the orange streaks from glowing airborne pyroclasts track to points below that suggest emission from multiple vents or an elongate vent with continuous extent, rather than a single point source, a topic returned to below in the context of an elongate trough developed on the MacKenney cone. That said, REW points out that it is hard to get a good idea of the scale from this video and that videos taken from other locations seem to show a wider, and at times two different fountain jets. Available video and photographic data has thus far prohibited estimating the width of the fountain at this stage of the eruption. REW (2013) citing Hetland (personal communication) and CONRED 856 noted that associated with these emissions the major tephra fall began, and it soon spread tens of kilometers to the N.

Early in the explosion on the 27th (exact timing unknown), a news team from a national television station (Notisiete) endured a shower of ballistics. REW (2013) noted that they were in the vicinity of Cerro Chino at probably less than 1 km from the vent, the zone with critical infrastructure most impacted (figure 53). Although most of the news team survived, reporter Anibal Archila’s death was apparently the result of direct impact from a large ballistic. His was the only icially confirmed death caused by the strong explosive phase. During a subsequent eruptive lull, a rescue team spent several dangerous hours in very close proximity to the vent, finding and rescuing missing people, and carrying out Archila’s body.

Figure 53. A truck parked directly N of the Pacaya’s active crater at Cerro Chino as seen in the aftermath of the 27-28 May explosions. Courtesy of Gustavo Chigna (INSIVUMEH).

Ballistics in excess of 0.5 m on their long axis fell at Cerro Chino and elsewhere within ~1 km of the vent area (figure 54). Some bombs on the ground reached sizes of 80 x 50 cm (Hetland personal communication) but part of that extent may have been due to splattering on impact. Farther away, the sizes of ballistics generally diminished with distance from the source. At Cerro Chino ballistic impacts broke concrete roofs, started fires in the radio shacks, and toppled antenna towers (REW, 2014; Wardman and others, 2010).

Figure 54. An example of a large bomb found in the near-source region. Courtesy of Gustavo Chigna (INSIVMEH).

When the intense phase started on the 27th, the evacuation of villages to the W (El Rodeo and El Patrocinio) was already underway. During the hours after the explosion’s onset on the 27th, more than 2,100 people were evacuated from the proximal villages to the town of San Vicente Pacaya (5 km NNW)(see related scenes in RT news, 2010 (b), table 1).

The settlements El Cedro, San Francisco de Sales, and Calderas, towns 2.5-3.5 km to the N, endured both ash as well as a dense barrage of hot ballistic bombs (figure 55). Many of the bombs were below 20 cm in diameter. Some of the ballistics pierced the corrugated (sheet metal or fiber cement) roofing common in Latin America. In some cases the ballistics also ignited fires that consumed most of the combustible contents of the buildings. Some roofs collapsed or buckled due to the load of deposited tephra.

Figure 55. Two photos taken soon after Pacaya’s 27-28 May eruptions illustrate the density of projectile penetrations through roofs of two large buildings in San Francisco de Sales (~3 km N of the MacKenney cone). Taken from REW (2013) with photo credit to Hetland.

The ballistics examined were of low density owing to vesicles larger than 1 mm in diameter. They contained sparse phenocrysts (often larger than 1 mm), most likely plagioclase (Hetland personal communication to REW and Hetland (2010).

REW (2013) noted that, from the observed damage to roofs in these villages, the density per unit area of impacts that pierced through the corrugated roofs averaged as high as on the order of 1 per square meter. Portions of the roofs in near-vent settlements also sustained many dents from bombs that delivered impacts with lower force. Although some communities were partially evacuated when many of the ballistics arrived, REW (2013) concluded that some residents remained within the communities and regions mostly affected.

Reports in Prensa Libre give insights into the scene of the evacuation and the barrage. Many of the residents evacuated on foot following narrow paths across the rugged rural terrain. Other residents remained behind in order to protect their belongings from theft. When the barrage came, those too close used whatever hard and resistant objects they could find to protect themselves, including hiding under furniture and using pots and pans to protect their heads. Some corroded metal roofs were weak prior to the eruption. Some people found refuge in buildings with heavier, concrete-slab roofs, which generally fared better.

Figure 56 shows an individual who clearly received medical attention, stitches, for a laceration on his forehead. According to REW (2013), Pacaya’s 2010 ballistic barrage caused more injuries than any recent eruptions. That said, data remain scanty on injuries rates and kinds, resultant disabilities, accident location, etc., although Wardman and others (2012) compiled some statistics.

Figure 56. Ballistic projectiles presented the most direct hazard from the 2010 explosions at Pacaya. This photo was found on the Boston Globe photo news site Boston.com (table 5). Their caption read, “A man shows the stitches he received after being injured by volcanic rock on the slopes of the Pacaya Volcano on May 28, 2010. (REUTERS/Daniel LeClair).” Courtesy of Boston.com.

The AmatitlÁn geothermal plant, located ~3 km N of the MacKenney cone to the N of San Francisco de Sales received ~20 cm of mostly lapilli-sized tephra. As Wardman and others (2012) noted, “Ballistic bombs and blocks also bombarded the plant, causing extensive damage to the plant’s roof and condenser fans. Fan blades were dented, bent and also suffered damage from abrasion. Minor denting of the intake and outlet pipe cladding was also reported however these impacts were superficial and did not require repair.” A photo showed cladding bearing multiple closely spaced dents on the side of a large pipe; the largest dent, 20 cm across, had ruptured through the sheet metal.

Post-explosion assessment of the MacKenney cone shed new light on the form and significance of the previously mentioned notch across it (a linear NW-trending trough passing through the summit, figure 57).

Figure 57. An annotated photo viewing the N side of the MacKenney cone in calm conditions at an unstated date following the May 2010 explosions. The prominent trough included a deep segment that had developed on the cone’s lower slopes (labeled ‘Possible crater’). During the 27-28 May 2010 eruption the trough appears to have served as an active fissure or series of vents emitting fountains (see figure 52 and related discussion). Courtesy of REW with photo credit to Gustavo Chigna.

The notch formed a prominent depression aligned both with the new SE-flank fissures and Cerro Chino cone on the outer NW crater rim. Portions of the RT video footage taken during vigorous stages of explosion suggests that at a paroxysmal stage of the explosion the trough served as an eruptive fissure emitting a vertically directed fountain as a curtain (table 5). REW (2013) also suggested that the eruptive fissure along the trough may have served as the vent for the ballistics that fell in previously mentioned settlements to the N.

The explosions broadest areal impact came from tephra fall. Figure 58 shows a close up of ash from a sample collected 22 km from the vent. Overall, the grain sizes ranged from sub-millimeter to centimeter size. An abundance of fine suspended particles in the air were not reported during or following the tephra fall.

Figure 58. Close up view looking at Pacaya tephra clasts collected in Guatemala City ~22 km NNE of the source. The smallest increments on ruler are in millimeters; the size range of grains here were mostly below ~ 3 mm diameter but grains under 0.2 mm were scarce to absent. The clasts consisted of black to dark brown vitric (crystal poor) scoria. Taken from REW (2013), who cited R. Cabria (personal communication).

As noted in table 1, in the afternoon on the 28th, high eruptive vigor resumed and tephra again fell on Guatemala City (CONRED 735). The ash fall on this day was lighter than on the 27th. Here aviation data (discussed below) did record the plume via satellite. The Washington Volcanic Ash Advisory Center (VAAC) noted (in their 6th advisory) an eruption in the afternoon on the 28th reaching (based on comparison of plume movement to modeling of winds aloft) ~13 km altitude.

During 29 May and onwards the intensity of volcanic activity decreased, with only relatively small eruptive plumes that occasionally produced minor tephra fall in the communities surrounding the volcano (CONRED 742). CONRED 748 noted that by the 29th, a total of 2,635 people were in shelters due to the eruption, with close to 800 home either damaged or destroyed. In the following days the attention of the emergency managers shifted from the eruption to the Tropical Storm Agatha, which had much broader extent and impact.

In the Pacaya and Guatamala City region, and along drainages carrying ash-charged run, both disasters combined. Lake AmatitlÁn rose, inundating low lying parts of the town with a water-and-ash mix (see photo documentation of impacts at Boston.com). Figure 59 is a photo taken ~12 km downstream of the Lake’s outlet.

Figure 59. The Pacaya tephra fall combined with storm run from Agnes led to swollen rivers in a ‘dual disaster.’ Those rivers formed new deposits along their beds from large amounts of in-swept debris, in this case including large boulders, trees, and a badly battered vehicle in the foreground. This press photograph was taken on 30 May 2010 as the flood water dropped. The location was the municipality of Palin, which sits along the Michatoya river downstream of Lake AmatitlÁn and ~10 km W of Pacaya. Taken from Boston.com with credit to Johan Ordonez/AFP/Getty Images.

Seismic record. INSIVUMEH and REW (2013) suggested a climax on the 27th starting shortly before 1800 local time and lasting ~40 minutes.

The seismic signal (figure 60, upper panel) contained a few scattered high amplitude events during the morning of 27 May 2010. Seismicity rose significantly about 1200 on the 27th, about doubling the RSAM values recorded during the previous 13 hours.

Figure 60. Seismicity recorded at Pacaya during the 2300 of 26 May through 1700 on 28 May (local times). The upper panel shows the seismic record and the lower panel shows the computed RSAM. Station PCG is a short-period seismometer located on Cerro Chino, ~1 km NW of Pacaya’s summit on the MacKenney cone. Courtesy of INSIVUMEH.

The first of about 10 strong peaks (seen on both the upper and lower panels of figure 60) took place around 1230 on the 27th. Those peaks represented a large escalation in seismicity an approximate doubling of the RSAM values. The highest peak on the record took place during 1730 to 1830 on the 27th, a ~6-fold increase in RSAM over the background values acquired earlier on the 27th. During the middle part of the 1800-1900 interval there was a peculiar several-minute-long period with low seismicity conspicuous on the seismic record (upper panel). After that, a series of closely spaced peaks of generally decreasing amplitude followed and then seismicity decreased substantially, particularly around 2300-2400 on the 27th. A second escalation of broadly similar size to the earlier one came on the 28th peaking at 1100 and then dropping.

In a later analysis of seismicity, Mercado and others (2012 correlated waveforms for 5 months before and 9 months after the May 2010 eruption. They noted that “No correlation was found between the events of each day during the five-month period before the eruption, thus, establishing no relationship with the periods of correlation found after the eruption. The post-eruptive sources of seismicity discovered were not active before the eruptive event of May 27, 2010, and therefore these sources must be strictly post-eruptive in nature.”

Aviation. Although there were 48 reports (Volcanic Ash Advisories, VAA’s or simply ‘advisories’) issued by the Washington Volcanic Ash Advisory Center (VAAC) on Pacaya behavior during the interval 27 May to 26 June 2010, weather clouds frequently masked the plume from the key satellite observation platform, the GOES-13 satellite. Where satellite observations of the plume were scarce or lacking, most of the VAA’s conveyed ground-based observations including media reports.

By the 3rd advisory, which was issued on the 28th, considerable ash had fallen at the International airport Aurora. There is some confusion as to the quantity of ash at the airport and over the region in general, but a photo on the 28th shows ash at the airport. Judging from ash load on the aircraft, the f walking just to the right of the aircraft, and adjacent tire tracks, the ash was on the order of ~1-cm thick (figure 61). This is in accord with INSIVUMEH’s summary report that said 5-7 mm of ash had fallen during the entire explosive 27-28 May eruption at the airport. This is also in accord with REW (2014), which discusses the complexities of assessing tephra thicknesses in more detail, and presents a preliminary isopach map that shows the S fringes of the Guatemala City urban area with 10 cm of ash and many parts of the urban area farther N, including the airport, with on the order of 1 cm of ash.

Figure 61. An American Airlines jet sits covered with ash from the Pacaya explosions at the International airport in Guatemala City on 28 May 2010. Runway cleanup took five days. The cleaning of the abrasive ash both destroyed the bituminous runway surface and all markings on it (Wardman and others, 2012). This photo was posted on the Boston Globe news website (Boston.com, see reference in table 5) with the credit to REUTERS/Daniel LeClair.

What follows is a summary of the advisories issued during 27 through 28 May (UTC).

The VAA’s frequently refer to the NAM (North American Mesoscale Model), a numerical model for short-term weather forecasting and in this case wind-velocity estimation. The model is run 4 times a day with 12 km horizontal resolution and with 1 hour temporal resolution, providing finer detail than other operational forecast models. An example of a model with less detail is the model called GFS (Global Forecast System), which predicts weather for many regions of the world, and was sometimes also used by the VAAC analysts.

The VAAC issued their 1st VAA for Pacaya during 2010 on May 27 at 1140 UTC, citing as key information sources GFS winds and INSIVUMEH. Eruption details noted small brief ash emissions near the summit at 1115 UTC. The ash cloud was not identifiable from the GOES-13 satellite owing to rain. The ash cloud was inferred to have remained low and near the volcano. GFS wind data suggested that for such a low ash cloud at that time, wind-directed transport would carry a plume S-SW and would only be significant for ~20 km. The analyst noted that eruption as then dominantly lava emission.

The 2nd advisory came out 7 hours later at 1845 UTC on the 27th indicating volcanic ash and gases to ~3.5 km altitude (noting ICAO as an information source). Ash was again not identifiable from the GOES-13 satellite owing to clouds.

The 3rd advisory, noting ‘ongoing emission of volcanic ash and gases,’ came out at 1257 UTC on the 28th, again lacking clear satellite identification of ash owing to clouds, in this case citing a thick tropical depression. This advisory relied on both a wind model (NAM winds) and an aviation meteorological report (a METAR). The advisory further noted media reports of ash on runways as discussed in the context of figure 61.

The 4th advisory was issued at 1554 UTC on the 28th, noting “increasing emissions” at 1515 UTC with INSIVUMEH reporting ash rising to 3.7 km altitude (FL 120) and spreading up to 27 km NW. Again, owing to extensive weather clouds, ash was again not visible from GOES-13 satellite.

The 5th advisory was issued at 1710 UTC on the 28th, noting “ongoing emissions” recorded at 1645 UTC. Plume has now become visible in [GOES-13] imagery and extends about 15 NMI [Nautical miles, 27 km] to the NNE of the summit. Plume top was at 3.7 km altitude (FL 120).

The 6th advisory was issued at 1915 UTC on the 28th, noting a large eruption recorded at 1815 UTC: “Large eruption seen to FL420 [42,000 feet, ~13 km altitude] based on NAM sounding for the area. Forecast winds remain mostly westerly to northwesterly. Winds at the time of observation blew the plume E at ~18 km/hr.

The 7th advisory was issued at 1930 UTC on the 28th (the last one that day); it repeated information about the eruption seen in imagery around 1815. In this advisory the wind was moving NW at 27 km/hr.

Slope stability study. Schaefer and others (2013) evaluated slope stability at Pacaya and commented on the possible implications of the trough across the MacKenney cone (figure 57). They consider the trough noted above as an example of a recent, smaller-volume collapse.

Specifically, they studied the SW flank of the edifice and developed a geomechanical model based upon field observations and laboratory tests of intact rocks from Pacaya. Their study included analysis of slope stability using numerical techniques and consideration of forces from gravity, magmatic pressure, and seismic loading as triggering mechanisms for slope failure.

Given the cone’s structural and seismo-tectonic setting, the likely magma pressures, and the history of past behavior, they suggested Pacaya lacked substantial gravitational stability.

Volcano Types

Tectonic Setting

Rock Types

Population

Within 5 kmWithin 10 kmWithin 30 kmWithin 100 km

4,250
53,579
2,454,482
7,033,094

Geological Summary

Eruptions from Pacaya, one of Guatemala's most active volcanoes, are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the ancestral Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate somma rim inside which the modern Pacaya volcano (Mackenney cone) grew. A subsidiary crater, Cerro Chino, was constructed on the NW somma rim and was last active in the 19th century. During the past several decades, activity has consisted of frequent strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and armored the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit of the growing young stratovolcano.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Synonyms

Pecul | San Juan de Amatitlán | Apacagua

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

Cerra, La

Cone

Chino, Cerro

Pyroclastic cone

2260 m

14° 23' 0" N

90° 36' 25" W

Grande, Cerro

Stratovolcano

2560 m

14° 23' 20" N

90° 35' 13" W

Mackenney

Stratovolcano

2552 m

14° 22' 52" N

90° 36' 4" W

Pacaya Viejo

Stratovolcano

Volcancito

Cone

Craters

Feature Name

Feature Type

Elevation

Latitude

Longitude

Amatitlán

Pleistocene caldera

Domes

Feature Name

Feature Type

Elevation

Latitude

Longitude

Chiquito, Cerro

Dome

2460 m

14° 23' 28" N

90° 35' 38" W

Photo Gallery

The Pacaya volcanic complex is seen here from the north. The rounded, forested lava dome of Cerro Grande forms the 2560 m high point at the left. The conical peak at the right is the historically active vent of Pacaya. It was constructed within an arcuate caldera whose rim forms the flat ridge on either side of the cone. This September 1962 photo was taken prior to a long-term eruption beginning in 1965 from a new vent on the west flank of the active cone. Frequent eruptions built MacKenney cone, which grew to the height of the previous cone.

Copyrighted photo by Dick Stoiber, 1962 (Dartmouth College).

A small eruption plume rises to the level of the rim of MacKenney crater in February 1967. This photo, taken from near Cerro Chino on the northern caldera rim, shows the morphology of the crater early in the long-duration eruption that began in 1965. More or less continuous explosive activity with periodic lava effusion first began on July 4, 1965 from a 250-m-wide pit crater that had formed in 1962 without eruptive activity, probably as the result of magma withdrawal.

Photo by Bill Rose, 1967 (Michigan Technological University).

A small spatter cone ejects incandescent bombs at the bottom of MacKenney crater in early February 1990. Activity had resumed at Pacaya in early January after a long quiescence following a major explosive eruption March 7-10, 1989. This eruption removed the upper 75 m of MacKenney cone and enlarged the 50-75 m wide crater to the 200 x 350 m widths seen in this photo.

Photo by Alfredo MacKenney, 1990.

A large hornito, or lava vent (center), grew on the NE flank of MacKenney cone (background peak) during a long-term eruption that began in 1965. This small NE-flank vent was active from August 5, 1984, until February 6, 1985, and produced a 50-m-high lava cone that fed sinuous lobes of lava down its flanks. Later eruptions buried most of the hornito before it was destroyed by an explosion in 1995. This photo was taken by Alfredo MacKenney, the Guatemala City physician and volcano enthusiast for whom the cone was named.

Photo by Alfredo MacKenney, 1985.

Spatter cones form when blobs of molten magma that are ejected from a vent by mild explosions solidify around the vent and form a steep-sided cone. This small spatter cone formed in MacKenney crater of Pacaya volcano in Guatemala on February 10, 1985. This was part of an ongoing eruption of Pacaya that began in 1965. At the time of this photo, the spatter cone was 6-m high, ejected incandescent volcanic bombs from its vent, and issued lava from its eastern (left) side.

Photo by Alfredo MacKenney, 1985.

MacKenney cone, the historically active vent of Pacaya volcano in Guatemala, was constructed within a horseshoe-shaped caldera produced by collapse of the summit of an ancestral volcano about 1100 years ago. The SW caldera rim forms the steep-sided scarp at the right, and the small knob halfway down the left-hand skyline is a remnant of the partially buried opposite rim. The blocky hill in the foreground is a hummock from the debris avalanche produced by the collapse. The avalanche traveled 25 km down to the Pacific coastal plain.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

More-or-less continuous mild strombolian explosions, such as the one producing the small plume above MacKenney crater at the right, have been punctuated by periodic larger explosions that eject incandescent bombs and large blocks. These trees on the Meseta, the rim of Pacaya's caldera, were stripped of vegetation by the larger explosions. The double cone on the horizon was constructed within this caldera, and MacKenney cone at the right has been constructed since 1965 on the flank of the older cone on the left horizon.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

Steam-and-ash clouds generated by collapse of the toe of a lava flow, incandescent in its upper portion, are seen on the SW flank of Pacaya volcano in November 1988. Frequent strombolian eruptions began at MacKenney cone in 1965 and were often accompanied by lava flows from summit or flank vents. Much of MacKenney cone, particularly the northern side, is armored by these basaltic lava flows, which have traveled up to 6 km down the southern flank.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

The less-frequently seen southern slopes of Pacaya volcano rise above foothills at the margin of the Pacific coastal plain of Guatemala. The twin-peaked summit seen here is MacKenney cone, which was constructed within a collapse scarp produced by the gravitational failure of an ancestral Pacaya volcano. The resulting debris avalanche traveled 25 km from the volcano to near the village of Las Chapernas on the coastal plain, beyond the point of this photo.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

Large-scale collapse of the summit of Pacaya volcano sometime between about 1550 and 600 years ago created a large horseshoe-shaped caldera. Collapse was followed by a large explosive eruption that produced widespread pyroclastic surges. This roughly 150-m-high section of the NW caldera wall exposes light-colored lava flows overlying pyroclastic deposits of the pre-collapse volcano. Subsequent eruptions have constructed a new cone within the caldera. Lava flows from MacKenney cone (out of view to the right) are slowly filling in the caldera moat.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

A sequence of thin lava flows forms the upper part of the NE caldera wall of Pacaya volcano. These basaltic lava flows were erupted during the final stages of an ancestral Pacaya volcano, which collapsed sometime between about 600 and 1550 years ago to form a large horseshoe-shaped caldera. The upper 50 m or so of the caldera wall is seen in this view with Cerro Grande, an older dome of Pacaya, forming the rounded peak at the upper right with the flank cone of Cerro Chiquito on the left.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

Cerro Chino, whose shallow crater rim can be seen at the left side of the long ridge cutting across the photo, was the site of a major eruption in 1775. This eruption began at vents on the SW flank of Cerro Chino (behind the crater in this view) and then migrated towards the summit of Cerro Chino. Powerful lava fountains were observed, and ashfall from this eruption, one of the largest in historical time at Pacaya, was reported up to 200 km away. Fresh black lava flows in the foreground from MacKenney cone fill the moat between it and Pacaya's caldera rim.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

Strombolian explosions such as this one in November 1988 are typical at Pacaya volcano. The incandescent traces of individual volcanic bombs that incrementally build MacKenney cone are seen in this time exposure. Cyclical activity lasting for several decades has consisted of long-term moderate explosive eruptions accompanied by periodic lava effusion that builds up the cone. This long-term moderate activity is punctuated by more infrequent larger explosions that destroy the upper part of the cone, after which the cone is reconstructed.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

Long-term strombolian eruptions began at Pacaya volcano in 1965 and continued for more than a quarter century. Nighttime incandescent explosions are often visible from Guatemala City, 40 km to the north. The accumulation of ejecta from frequent strombolian eruptions periodically raises the height of MacKenney cone after it has been partially destroyed by intermittent larger explosions. This November 1988 photo also shows a lava flow from a fissure on the west flank of MacKenney cone descending the right-hand skyline.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

The hilly terrain in the foreground is part of a debris-avalanche deposit that extends 25 km from the summit of Pacaya volcano to the Pacific coastal plain. Portions of the avalanche rode more than 100 m up the side of the ridge in the background before coming to rest on the valley floor. The avalanche was accompanied by a pyroclastic surge whose deposits overlie the proximal 10 km of the avalanche deposit. The collapse is bracketed by tephra layers dated at about 1550 and 600 years ago.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

These fine-grained ash layers and coarser laminated scoria-bearing layers were deposited by pyroclastic surges following collapse of the summit of Pacaya volcano. This surge deposit overlies the avalanche deposit and drapes topography in a 90 degree arc as far as 10 km SW of the summit. The timing of this major event is bracketed by tephra layers dated at about 1550 and 600 years ago.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

One of the largest historical eruptions of Pacaya volcano, illustrated in this contemporary painting, took place in 1775. An explosive eruption began on July 1, 1775 when several vents opened on the SW flank of Cerro Chino. A lava flow traveled to the south, eventually reaching 1000 m altitude. The vents migrated towards the summit of Cerro Chino; one reference refers to activity at the summit 22 days into the eruption. Ashfall from this eruption was reported up to 200 km away.

Fresh unvegetated lava fields armour the NW flank of Pacaya volcano. Two prominent flow fields originating from flank vents can be seen in this 1987 photo. The cone on the lower left horizon is a large spatter cone that formed in 1981. Lava flows from subsequent eruptions have nearly buried this cone. The forested slopes at the left are below Cerro Chino, a cinder cone on the rim of Pacaya's large horseshoe-shaped caldera.

Photo by Klaus Mehl, 1987 (GEOMAR, Germany).

A lava flow issues from a WNW-flank vent on MacKenney cone at Pacaya volcano on January 10, 1987. Steam pours from the summit crater, which was producing strombolian eruptions at the same time. Comparable activity at Pacaya began in 1965 and continued into the next century.

Photo by Klaus Mehl, 1987 (GEOMAR, Germany).

Vegetation-free MacKenney cone filling the upper left-hand quadrant of the photo was constructed within a large 3 x 6 km wide caldera. This feature, whose eastern and northern rims are visible at the right-hand and top sides of the photo, respectively, was formed as a result of massive slope failure of an ancestral Pacaya volcano. Lava flows from MacKenney cone traveled to the west and south beyond the breached caldera. The circular crater at the top center is Cerro Chino, a cinder cone constructed on the NW rim of the caldera.

Aerial photograph by Instituto Geográfico Nacional, 1981.

Incandescent volcanic bombs ejected from MacKenney crater at Pacaya volcano fill most of the frame of this 1968 photo. Long-term strombolian eruptions began at MacKenney crater in 1965 and continued for several decades.

Photo by William Melson, 1968 (Smithsonian Institution).

A towering eruption column rises above Pacaya volcano, seen here in 1987 from the NW along the highway to Antigua Guatemala. A column of steam and ash rises above powerful lava fountains. Continuous strombolian eruptions from Pacaya that began in 1965 were periodically interrupted by larger explosions such as this one, which deposited ash across wide areas.

Anonymous photo courtesy Norm Banks (U.S. Geological Survey), 1987.

MacKenney crater (right center), constructed NW of an older cone at the left, has been the source of most of Pacaya's historical eruptions. Frequent strombolian eruptions repeatedly build up the cone after it is partially destroyed by intermittent larger explosions. An extensive field of black lava flows produced from MacKenney cone since 1965 appears in the background of this January 25, 1987 view from the NE.

Photo by Norm Banks, 1987 (U.S. Geological Survey).

An incandescent lava flow, accompanied by strombolian explosions from the summit crater, travels down the SW flank of MacKenney cone in this November 1994 nighttime photo. Pacaya has displayed more or less continual eruptive activity since 1965.

Copyrighted photo by Stephen O'Meara, 1994.

Steam pouring from the summit crater of MacKenney cone descends its SW flank in this November 10, 1994 aerial view of Pacaya. After powerful explosions March 7-10, 1989 that destroyed the upper 75 m of MacKenney cone, strombolian eruptions resumed in early January 1990. Explosive eruptions were accompanied by lava flows that again armored much of the cone.

Copyrighted photo by Stephen O'Meara, 1994.

Strombolian eruptions at Pacaya volcano in Guatemala produce a colorful nighttime display. This November 1988 time exposure traces the incandescent parabolic arcs of individual volcanic bombs explosively ejected from the vent. Larger bombs remain incandescent after they hit the surface of the cone and roll down its flank. The orange line at the lower right is a lava flow that issued from a fissure on the upper NW flank of MacKenney cone.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

Thin wisps of steam rise from fumaroles at Laguna Caldera, within Amatitlán caldera. In addition to the fumaroles seen here near the southern margin of the caldera, hot springs are located at several places along the shore of Lake Amatitlán in the center of the caldera. This largely Pleistocene caldera has been the site of geothermal exploration because of its recent large silicic eruptions (the latest less than 23,000 years ago) and residual high heat flow.

Photo by Pat Dobson, 1997 (Lawrence Berkeley National Laboratory).

Well AMF-1 within Amatitlán caldera is seen here with the Laguna Caldera fault scarp in the background. Geothermal development within this 14 x 16 km wide Pleistocene caldera will provide electrical power to Guatemala's capital city, which overlies deposits from Amatitlán caldera.

Photo by Pat Dobson, 1997 (Lawrence Berkeley National Laboratory).

Thick units of the 84,000-year-old Los Chocoyos Ash are exposed south of Guatemala City, more than 100 km from its source at Atitlán caldera. Three flow units are visible here. The pinkish layer at the center of the outcrop is the oxidized top of the pyroclastic-flow deposit and is one cooling unit. The bottom two layers are the top and bottom halves of the thick white layer of the pyroclastic-flow deposit. The two fall deposits above the Los Chocoyos Ash are unit E from Amatitlán caldera and the younger unit C from Agua volcano.

Photo by Bill Rose, 1978 (Michigan Technological University).

Steam pours from incandescent spatter cones that fed lava flows blanketing the floor of MacKenney crater of Pacaya volcano. In May 1981, at the time of this photo, the broad several-hundred-meter wide crater rim lay well below the summit of the cone. From May 9 to June 2 lava flows from the spatter cones covered the crater floor and spilled over a notch in the NW crater rim. In the background are the twin volcanoes of Fuego and Acatenango (right-center) and the conical Agua volcano (right).

Photo by Bill Rose, 1981 (Michigan Technological University).

In February 1972 a lava flow from vents at the base of MacKenney cone descended the western flanks and covered farmlands on the caldera floor below the cone. The flow divided around topographic irregularities, forming several large kipukas. Lava effusion began on February 2 and lasted until the 27th. This December 1972 photo was taken from the summit of MacKenney cone with the village of El Patrocinio at the upper right, beyond the lava flow.

Photo by Bill Rose, 1972 (Michigan Technological University).

A spectacular aerial view from the east shows four major volcanoes located within 45 km of Guatemala City. In the foreground is Pacaya, which has been in frequent eruption since 1965. The modern cone (left-center) partially overlaps the rim of a large caldera. The massive conical volcano at the right-center horizon is Volcán de Agua, the only one of the four not to have erupted in historical time. The twin volcanoes of Acatenango (right) and Fuego (left) lie behind Agua. These latter three volcanoes overlook the historical city of Antigua Guatemala.

Photo by Bill Rose, 1991 (Michigan Technological University).

Spectacular grooved lava surfaces were formed when still-fluid lava was squeezed through an irregular crack in previously solidified crust. Note the rock hammer at the right-center for scale. Frequent lava extrusion has occurred at Pacaya since 1965.

Photo by Bill Rose, 1978 (Michigan Technological University).

In May 1981 lava flows spilled over a notch in the NW rim of MacKenney crater and flowed down the north flank. This view from Meseta on the caldera rim shows multiple individual flow lobes that traveled within prominent levees. The dark lava field at the right contrasts with ash-covered slopes of the cone on the left. This period of frequent lava extrusion lasted from May 9 until June 2.

Photo by Bill Rose, 1981 (Michigan Technological University).

The Volcán de Pacaya massif rises above skyscrapers of the capital city of Guatemala, located only 30 km to the north. The rounded, forested lava dome of Cerro Grande forms the 2560 m high point at the left. The twin peak at the right is the historically active vent of Pacaya, with the right-hand summit being MacKenney cone, which has been active since 1965. The modern cone was constructed within an arcuate caldera whose rim forms the ridge on either side. Eruptions of Pacaya are often visible from Guatemala City.

Photo by Bill Rose, 1989 (Michigan Technological University).

Blobs of incandescent magma are ejected from a spatter cone in MacKenney crater in February 1981. Lava flows from the cone filled the crater floor and spilled over a notch in the crater rim.

Photo by Bill Rose, 1981 (Michigan Technological University).

An incandescent lava fountain rises from MacKenney crater, which at this point, early in the 1965-1989 eruption, lay well below the summit of the cone. The date of the photo is not certain, but may have been around 1967. This view looks west from the summit of MacKenney cone with the slopes of Agua volcano in the right distance.

Glow from strombolian eruptions at a cone in MacKenney crater is reflected off the back wall of the crater. A lava flow spills through a notch in the breached crater and descends the northern flank. The date of the photo is not certain, but may have been around 1967.

The toe of the 1972 lava flow ends on grassy slopes on the southern flank of MacKenney cone. Broken slabs mark the flow front. The rock hammer at the upper right provides scale next to a spectacular grooved lava surface on the top of the flow front that formed when still-fluid lava was squeezed through an irregular crack in previously solidified crust.

Photo by Bill Rose, 1972 (Michigan Technological University).

Dark-colored lava flows erupted in late 1998 blanket the upper left-hand flank of smoking MacKenney cone in February 1999. The flows originated from vents at the summit and upper flanks of the cone and descended into the caldera moat of Pacaya before being deflected by the caldera wall to the west. The hill on the left horizon is Cerro Chino, a cinder cone constructed on the NW rim of the caldera. Strong explosive eruptions in 1998 created a notch in the summit of MacKenney cone and the deep gully that extends diagonally down to the right.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Frequent eruptions from MacKenney crater have kept the flanks of the volcano free of vegetation. The summit of the cone towers more than 1000 m above its base. The darker area at the lower left-center in this February 1999 photo is a September 1998 lava flow from a vent on the lower SW flank at about 1800 m elevation that bifurcated into lobes that traveled to the SW and south. The lighter-colored area below the flow extending to the right margin of the photo is a 2-km-long debris-avalanche deposit formed when the summit crater rim collapsed.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Steaming MacKenney cone is seen here from the NW below Cerro Chino cinder cone, whose ash-mantled slope appears at the left. Dark-colored lava flows that were erupted in 1998 descend from the summit and cascade down from the caldera moat at the left. This flow and the two dark lobes above it originated from MacKenney cone during the September 18-19, 1998 eruption. Light-colored tephra deposits between the flows mantle previous lava flows.

Photo by Paul Kimberly, 1999 (Smithsonian Institution).

Lava flows erupted in September 1998 cover the floor of the caldera moat of Pacaya volcano. The caldera wall here is about 100 m high. Frequent lava flows from MacKenney cone, whose slopes are visible at the right, have been gradually filling the caldera moat, whose floor is many tens of meters higher than it was a decade ago. Eruptions from MacKenney cone have been more-or-less continuous since its formation in 1965. The ridge in the foreground is the crater rim of Cerro Chino.

Photo by Paul Kimberly, 1999 (Smithsonian Institution).

A steam plume rising from a geothermal site in Amatitlán caldera is seen here from the caldera rim of Pacaya volcano with Guatemala City in the background. Laguna Calderas is the lake to the right of the plant, and beyond the ridge at the left is part of Lake Amatitlán. The 14 x 16 km wide Amatitlán caldera produced a large number of major explosive eruptions that blanketed the current site of Guatemala City with pyroclastic flows during the late Pleistocene. The caldera retains a high heat flow that is being exploited for geothermal energy.

Photo by Paul Kimberly, 1999 (Smithsonian Institution).

Of the four major Guatemalan volcanoes in this photo, only conical Agua volcano (right-center horizon) has not erupted during historical time. Lava flows from MacKenney cone (forming the slope in the left foreground) have filled in the moat of the caldera of Pacaya volcano almost to the level of the lower crater rim of Cerro Chino (right-center foreground), whose summit bristles with communication antennas. The twin volcanoes on the left horizon are Fuego (left), one of the most active in Guatemala, and Acatenango (right).

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Scoria deposits from periodic larger eruptions of Pacaya volcano blanket the roof of a structure on Cerro Chino. A major eruption on June 7, 1995 caused the WNW side of Pacaya's crater to collapse, producing debris that slammed into the caldera wall at Cerro Chino, 1 km NW of the summit. A secondary hot cloud swept over Cerro Chino, destroyed a radio antenna, and affected houses within 2 km of the active vent. The shockwave threw INSIVUMEH observer Pastor Alfaro down a slope, fracturing his leg.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Thick sequences of tephra deposits from Amatitlán caldera are abundantly exposed in roadcuts in the Guatemala City area. This exposure, with INSIVUMEH geologist Otoniel Matías for scale at the lower right, is located south of the capital city, along the road to Palin. Major explosive eruptions from Amatitlán caldera have been dated between about 300,000 and less than 23,000 years ago. The northern caldera rim is buried by thick pyroclastic deposits and underlies portions of Guatemala City.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Vigorous lava fountaining that initially reached heights of 800 m but diminished to 300 m occurred during a major eruptive episode the afternoon of January 16, 2000. Fallout from the fountains fed near-constant nuées ardentes and lava flows that traveled to the SW and north. Typical strombolian activity resumed around 2030 hrs. The January 16 episode was one of the most spectacular at Pacaya in its current 35-year-long history of eruption. More or less continuous activity had resumed at Pacaya in January 1990 after a 9-month-long quiescence.

Photo by Gene West, 2000.

An incandescent lava fountain rises above MacKenney crater during a major eruptive episode on January 16, 2000 against a backdrop of ash-rich and ash-poor eruption clouds. The fountaining feeds a lava flow that descends the northern flank of the cone. After a quiescent period of nine months, eruptive activity had been more or less continuous at Pacaya since January 1990. Activity has consisted of frequent moderate eruptions that build up the summit cone, punctuated by occasional larger explosions such as this one that enlarge the crater and reduce the height of the cone.

Photo by Gene West, 2000.

This dramatic contemporary painting of the 1775 eruption of Pacaya volcano shows a lava fountain and eruption plume rising above Cerro Chino. The perspective of the painting is from Meseta on the NE rim of Pacaya's large summit caldera. At the left is the conical peak now known as MacKenney cone, and at the far right is Agua volcano. An explosive eruption, one of the largest in historical time from Pacaya, began on July 1, 1775 and produced extensive ashfall and a lava flow that eventually reached 1000 m altitude on the south flank.

An aerial view from the NNW shows steaming MacKenney cone. The rim of the large horseshoe-shaped caldera within which the cone was constructed is visible in the foreground, partially overtopped by Cerro Chino cinder cone on the right. The caldera was formed by collapse of Pacaya during the late-Holocene and produced a debris avalanche that swept down the Metapa river drainage to the SE, reaching as far as the Pacific coastal plain, 25 km away.

Copyrighted photo by Steve and Donna O'Meara, 1999.

A prominent notch cuts the steaming summit crater of MacKenney cone on Pacaya in this February 1999 aerial view from the north. A trail in the foreground tracks the rim of a large horseshoe-shaped caldera inside which the cone was constructed. Lava flows from the summit crater and flank vents of MacKenney cone have poured into the caldera moat and subsequently been deflected to the west and south.

Copyrighted photo by Steve and Donna O'Meara, 1999.

Steaming Pacaya volcano (lower right) lies across a valley from symmetrical Agua volcano (upper left). Pacaya was constructed near the southern margin of Amatitlán caldera, whose SE rim lies near the right-center margin. The 14 x 16 km wide caldera was formed during a series of major silicic explosive eruptions between about 300,000 and 23,000 years ago. The irregular margins of Lake Amatitlán are constrained on the SW side by post-caldera lava domes. The outskirts of Guatemala City lie at the upper right.

The distal end of the massive Escuintla debris-avalanche deposit is defined by the purple-colored area bounded on its SW side by the arcuate Río Naranjo at the bottom of the image. The avalanche traveled about 50 km from its source at Fuego-Acatenango complex (upper left), covering an exposed area of about 300 sq km on the Pacific coastal plain. Another debris avalanche from Pacaya volcano traveled about 25 km SW to the purplish area at the base of the Guatemalan Highlands. Guatemala City lies at the extreme upper right.

A volcanic plume extends to the east from MacKenney crater at Pacaya volcano in this December 8, 2000 Landsat image. The unvegetated modern cone was constructed within a scarp (visible south of the summit) left by a major edifice collapse at Pacaya volcano about 1100 years ago. The avalanche swept 25 km down the Río Metapa drainage (lower left) to the Pacific coastal plain. Pacaya was constructed near the southern rim of Amatitlán caldera, partially filled by Lake Amatitlán at the top right.

Nighttime (time-lapse) view of Pacaya's MacKenney cone as seen looking W in December 2007. Flat-topped, antenna-laden Cerro Chino of the Pacaya complex is at lower right, and at distance in background from right to left reside Agua, Acatenango, and Fuego stratovolcanoes.

Photo courtesy of Richard Roscoe (www.photovolcanic.com).

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.

EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).